Network Working Group W. Eddy
Internet-Draft Verizon
Intended status: Experimental L. Wood
Expires: September 12, 2008 Cisco Systems
W. Ivancic
NASA
March 11, 2008
Checksum Ciphersuites for the Bundle Protocol
draft-irtf-dtnrg-bundle-checksum-02
Status of this Memo
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Abstract
The Delay-Tolerant Networking Bundle Protocol includes a custody
transfer mechanism to provide acknowledgements of receipt for
particular bundles. No checksum is included in the basic DTN Bundle
Protocol, however, so at intermediate hops, it is not possible to
verify that bundles have been either forwarded or passed through
convergence layers without error. Without assurance that a bundle
has been received without errors, the custody transfer receipt cannot
guarantee that a correct copy of the bundle has been transferred.
This document attempts to address the situation by defining new
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ciphersuites for use within the existing Bundle Security Protocol's
Payload Integrity Block (formerly called the Payload Security Block)
to provide error-detection functions regardless of an
implementation's support for other, more complex, security-providing
ciphersuites. This creates the checksum service needed for error-
free reliability, but does so at the expense of divorcing security
concerns from the few new reliability-only ciphersuite definitions
that are introduced here. This document lengthily discusses the pros
and cons of this approach and the existing constraints that combined
to drive this design.
Table of Contents
1. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Overview of Header and Payload Integrity . . . . . . . . . 4
2. Use of the Payload Integrity Block . . . . . . . . . . . . . . 7
2.1. Differences from Intended Use of the Payload Integrity
Block . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. INSECURE Ciphersuites . . . . . . . . . . . . . . . . . . . . 10
3.1. Generation and Processing Rules . . . . . . . . . . . . . 12
4. Performance Considerations . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Mandatory BSP Elements Needed to Implement
Error-Detection . . . . . . . . . . . . . . . . . . . 18
A.1. Mutable Canonicalization . . . . . . . . . . . . . . . . . 19
A.2. PIB Format . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Motivations
Reliable transmission of information is a well-known problem for all
protocol layers. Error-detection and correction capabilities are
frequently found in lower layers, but are also present in many
higher-layer protocols in order to detect residual bit errors and
bugs that introduce errors. For example, IPv4 verifies a simple
header checksum before processing inbound packets, even when running
over a data link, such as Ethernet, that already performs a stronger
CRC. The TCP and UDP transport protocols further include a checksum
covering their payloads as well as some IP header fields. What may
seem like paranoia is actually not unfounded, as errors in received
data or packet corruption are known to creep into networking systems
from many causes other than channel noise [SP00]. Although coding of
data on the channel can reduce the impact of channel noise, and CRCs
across each link in a network path can catch many channel-induced
errors, end-to-end checksums across the entire network path are
understood to be necessary for applications requiring certainty that
the data received is error-free [SGHP98].
The Delay/Disruption-Tolerant Networking (DTN) architecture [RFC4838]
is founded on an overlay of Bundle Agents (BAs). These Bundle Agents
forward data units called bundles via the Bundle Protocol [RFC5050].
Bundles may be lost or errored both during transmission between BAs,
or within a BA itself. Bundles belonging to applications that are
not tolerant of lost data have a "custody transfer" flag that
requests reliable transmission between bundle agents. The notion of
reliability used in the basic custody transfer mechanism does not
take the integrity of bundles into account, but acknowledges a
bundle's receipt and transfers its custody without verifying its
internal data integrity. In this way, the bundle protocol provides
what has been called a "better than best-effort" service, which
attempts through persistence to provide delivery of content without
making claims about the end-to-end integrity of that content.
However, [RFC4838] discusses 'reliable delivery', this is expected to
be provided to the bundle layer, and is not a property of the bundle
layer. The "convergence layer adapters" that connect BAs to each
other may or may not detect and correct errors before presenting
bundle data to the BAs themselves. Convergence layer error detection
and correction may be adequate in many cases, but is not always
sufficient, and the insufficiency is encapsulated in the well-known
end-to-end principle [SRC84]. It is possible (and even statistically
likely) that either transmission errors will go unnoticed, or
unchecked errors will be introduced within a BA's memory, storage, or
forwarding systems. Here, each convergence-layer check is analogous
to a data-link check, covering a single hop between Bundle Agents,
but not the entire network path between source and destination for
the bundle.
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Within the context of DTN, even stronger convergence-layer adapter
error detection is not sufficient. Errors within a BA's device
drivers, errors due to memory issues within the BA's host, e.g.
radiation-induced soft errors, and errors introduced from file-system
corruption cannot be detected by convergence layer adapters, as these
errors occur in gaps between successive phases of forwarding and
convergence-layer processing. In order to ensure integrity of DTN
bundles forwarded across a system composed of BAs and convergence
layer adapters, end-to-end computation and verification of checksums
is required [SRC84]. To detect errors introduced in storage, a
checksum across the bundle could be verified before the bundle is
sent to the next hop.
Within this document, we describe a use of the Bundle Security
Protocol (BSP) [I-D.irtf-dtnrg-bundle-security] in order to provide
the desired error-detection service by defining suitable BSP
ciphersuites. The design decisions for doing this are painstakingly
explained in Section 2. It should be clearly understood by readers,
implementers, and users that we are not using the BSP in a way that
provides any level of security, which we explain fully in
Section 2.1. The guarantee that we attempt to provide is that
specific blocks within a received bundle are highly likely to have
been propagated across the overlay without errors, under the
assumptions of no malicious activity within or between Bundle Agents
and no capability to inject forged bundles. The actual format and
use of this error-detection mechanism based on the BSP and
requirements for support are described in Section 3.
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]
1.1. Overview of Header and Payload Integrity
It is worthwhile to review checksum use in the Internet to ensure
error-free delivery, as this should provide a familiar starting point
for the reader. This can then be used as a basis for thinking about
error-free delivery of bundles. We must distinguish between the need
to verify the integrity and reliability of ("protect") carried
payloads, and the need to verify the reliability of the protocol
carrying the payload - i.e. the need to also protect the header
metadata surrounding the payload that the protocol tells itself.
Within the topic of payload reliability, applications have differing
requirements. Some applications are capable of dealing with errored
content, and desire the delivery of any sent content blocks, even if
they are not entirely the same content blocks that were originally
transmitted. For instance, some voice and video codecs degrade
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perceptibly under loss of content, but cope well with low levels of
error within delivered content. To date, however, most Internet
applications have not been tolerant of errored content.
In IPv4, header and payload reliability are implemented separately,
with the IPv4 header checksum covering the header information, while
the TCP or UDP checksum covers the payload. The TCP/UDP checksum
also covers certain important IP header fields - this is the 'pseudo-
header' check. In IPv6, the header checksum was removed, to speed up
router processing as header fields were changed. The 'pseudo-header'
checksum of important IPv6 header fields is still there as part of
the overall payload checksum implemented by TCP, UDP, and other
protocols. This upper-layer pseudo-header checksum combines the
header and payload checks efficiently at the receiving IPv6 endhost
to prevent misdelivery of packets and protect against corrupted
payloads. This checksum is a final end-to-end check across the
entire IP delivery path - for both header and payload. It ensures
that the payload is delivered to the right place, without corruption
being introduced.
The TCP and UDP checksum mechanism is frequently criticised for being
weak in that there are classes of errors it does not detect. This
checksum is computed by summing 16-bit values within a packet. If
two strings are swapped in position within the packet, the checksum
can remain unchanged even though the datagram is now different from
the original, and clearly corrupted. The UDP convergence-layer
adapter that has been popularly implemented in DTN stacks relies on
UDP's usual 16-bit one's-complement checksum (the same algorithm used
for IPv4 headers and TCP segments) to validate incoming datagrams.
The proposed TCP-based convergence layer [I-D.irtf-dtnrg-tcp-clayer]
relies on the same checksum algorithm. This checksum algorithm is
remarkably useful in its position covering an entire network path and
ability to detect all errors introduced, even though its overall
strength against errors is considered weak, and in more recent
transport protocols has been reconsidered; for instance, SCTP uses
the CRC-32c algorithm instead [RFC3309]. It is the ability to
provide a check that the packet at the destination is the same at the
source that is most useful for Internet traffic; overall strength is
secondary to this. The UDP- and TCP- convergence layers and their
checksums are only useful for one 'bundle hop' between peer agents
across the Internet, and will not cover multiple bundle hops as an
end-to-end checksum.
In absence of IP packet fragmentation, it would be possible for all
intermediate routers in the terrestrial Internet to verify the upper-
layer pseudo-header checksum at each hop rather than just checking
data-link CRCs, but this is not normally done, as it would be
burdensome to busy routers as well as a philosophical "layer
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violation". (Network Address Translation, where the change in
pseudo-header addresses affects the checksum value, is an exception
to this.) Not checking the payload at every hop takes advantage of
the closed-loop nature of fast Internet communication; the cost of
packet delivery is cheap, so necessary protection against corrupted
packets or headers need only be done at the last possible moment with
a check at the endhost, as the costs incurred in delivering the
packet to that node, and any resent packets to that node, are
minuscule. It is the cheap cost of resending and the tight end-to-
end control loop which can permit reliability checks to be pushed up
to or only done at the application layer, in accordance with the end-
to-end principle, without adversely affecting overall communication
efficiency.
In challenged delay-tolerant networks, the network capacity,
forwarding and storage costs for bundles are far higher, as are the
costs of getting a resent bundle. This produces a longer, slower,
control loop and alters the cost/benefit tradeoff. It becomes
worthwhile for intermediate bundle nodes to check non-error-tolerant
content, which includes important header fields and metadata, for
error-free correctness before forwarding it, to prevent incurring
unnecessary transmission costs and to also increase the possibility
of getting the bundle from the source or from over the previous hop
again if it needs to be resent, shortening the overall resend time.
Due to the more open-loop nature of communication across delay-
tolerant and disrupted networks, applications do not communicate as
well or as easily as they do in the traditional Internet, meaning
that the efficiency tradeoff for application-only checks is such that
non-error-tolerant DTN application performance can be expected to
increase with earlier detection of errors and earlier resends within
the network. Network costs are decreased and utilization is
increased by early discard of known 'bad' traffic. This is a concern
that has previously led to e.g. ATM Early Packet Discard (EPD).
This shift in costs caused by the difficulty of resending
information, when compared to the traditional Internet, makes
verifying the correctness of the header and non-error-tolerant
payload content at each bundle node much more important, to permit
faster resends and conserve storage and transmission capacity. The
Bundle Protocol, as specified in [RFC5050], does neither of these
things. Nor does the Bundle Protocol protect itself end-to-end.
There is no Bundle Protocol check on header or payload at destination
bundle nodes; there is no end-to-end equivalent to the two checks
that the TCP/UDP checksum so neatly combines and provides for
Internet traffic. There is a need for a check at intermediate nodes
in challenged networks for suitable traffic, given the cost tradeoffs
outlined above, yet those nodes cannot take advantage of a
precomputed end-to-end checksum unless one is first provided for
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them.
This document lays out an unsatisfying way to redress this lack of
robustness in the existing Bundle Protocol in Section 3, and
carefully describes the drawbacks to this method in Section 4 and
Section 5.
2. Use of the Payload Integrity Block
The BSP defines three types of blocks:
o the Bundle Authentication Block (BAB),
o the Payload Integrity Block (PIB),
o the Payload Confidentiality Block (PCB).
Algorithms applied within each of these blocks could be reused to
detect errors introduced in bundle contents. However, based on the
different design goals motivating these three block types, the PIB is
the only candidate that is truly suitable for supporting the type of
checksum fields needed to yield end-to-end reliability of received
bundles.
The BAB is intended to operate along a single hop within a DTN
overlay network, and due to the issues discussed in the previous
section, even an end-to-end chain of hops using the BAB over each hop
is not sufficient for ensuring reliability.
The PCB is primarily concerned with the operation of privacy
transforms over a bundle's contents, which are intended to
significantly alter and disguise the protected data while in transit,
rather than simply performing consistency checks over untransformed
data.
The PIB is intended to be used end-to-end; that is, by a set of
endpoints, rather than hop-by-hop at each intermediate point. The
PIB is intended to be used with sets of cryptographic algorithms
(ciphersuites) that provide Message Authentication Codes (MACs) or
signatures over bundle or block contents. MAC and signature
algorithms are security constructions that may allow verification of
a legitimate sender (authentication), detection of in-transit
tampering (integrity), and proof of a particular sender (non-
repudiation). As a consequence of the integrity goal, which is based
on the assumption of an adversary that can alter messages in-flight,
MACs and signatures can also be effective at detecting errors that
occur without the presence of an attacker and in the absence of any
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malicious intent (e.g. due to bit errors within transmission media,
file system corruption, etc.). Since the PIB uses the BSP's mutable
canonicalization and covers the Primary Bundle Block, the EIDs and
other data influencing forwarding and delivery of payloads are also
protected by the MACs or signatures in addition to the payload data.
The error-detecting and rejecting capabilities of a MAC or signature
are similar to those of more-simple checksum algorithms that are
intended only for error-detection. In fact, several popular MAC and
signature constructions use checksums as primary components. For
instance, the MD5 hash (checksum) algorithm [RFC1321] is used within
the HMAC-MD5 keyed-hash MAC construction [RFC2104]. Computationally,
for large messages, the efficiency of a security construction
providing integrity is similar to that of a simple checksum, although
for short messages, it may be much worse. For instance, HMAC
requires multiple applications of the underlying hash function, with
the final one being over a very short input, but if the message
itself fits within a single block, this results in twice the overhead
compared to a simple checksum. Thus, assuming large bundles in
relation to the block size of typical hash functions, the PIB can
provide end-to-end error-detection capability for bundles from the
standpoints of both reasonable effectiveness and reasonable
computational cost.
2.1. Differences from Intended Use of the Payload Integrity Block
The main difference between any simple error-detecting checksum and a
security construction designed for integrity is that the security
construction requires keying material. Key management is recognized
as an outstanding problem within the DTNRG
[I-D.irtf-dtnrg-sec-overview] and thought to be quite difficult
[I-D.farrell-dtnrg-km]. Key management in well-connected systems,
such as the Internet, is difficult itself, without the additional
complications of a DTN networking environment. However, if using a
keyed security construction for simple error-detection, the secrecy
of the key is unimportant, and a feasible approach is to specify a
hard-coded key that all nodes use in the error-detection mechanism
(e.g. the NULL ciphersuite in LTP's authentication extension is one
example [I-D.irtf-dtnrg-ltp-extensions]). Using this approach,
existing keyed ciphersuites defined for the PIB could be used with
NULL keys to provide an error-detection mechanism, without requiring
a key management mechanism. However, this key-based method reuses a
security mechanism for error detection, which it is not designed for.
As the Bundle Protocol has no separate outer error detection covering
this security payload, if a private key is used, then third-party
intermediate nodes that do not possess that private key cannot
determine the reliability of the content, and would be unable to
prevent unnecessary forwarding of errored bundles belonging to non-
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error-tolerant applications. This may lead to decreased performance
of the network due to utilization of storage, bandwidth, and power
for unusable bundles, and poor end-to-end performance due to the open
nature of its control loop as discussed above.
If early detection and discard of unusable bundles is done to prevent
errors, this gives unencrypted bundle delivery a network performance
advantage over secure bundle delivery with private keys.
Applications wanting both security and network performance would gain
both by implementing their own end-to-end security within unencrypted
bundles using the insecure ciphersuites defined in this document.
The only PIB ciphersuite included in the BSP to date is PIB-RSA-
SHA256, which creates and verifies signatures of bundles using RSA
asymmetric-key operations over a SHA-256 hash of the bundle. The
length of the SHA-256 output is excessive for providing simple error-
detection on all but extremely large bundles and the overhead of SHA-
256 and RSA calculations is also disadvantageous. It would also
require some small change in either security block metadata bits or
the block type field to signal that the special (unsecure) NULL key
pair for error-detection only was to be used with PIB-RSA-SHA256.
But the biggest problem with PIB-RSA-SHA256 is the bulk of code
needed to support the RSA operations, which include math on numbers
larger than that supported by common processors' native instruction
sets and modular arithmetic libraries. Since error-detection and
rejection is a vital and absolutely essential component of reliable
networking protocols, and much of the purpose of the DTN architecture
is to enable internetworking of devices with limited resources, e.g.
motes, it would be burdensome on limited low-end embedded systems to
require all Bundle Protocol implementations to include RSA code.
The BAB-HMAC ciphersuite that uses SHA1 [RFC3174] within the HMAC
construction (HMAC-SHA1) has been specified as mandatory for BSP
support. Even though the BAB is not appropriate for end-to-end
error-detection, it is certain that BSP implementations will include
HMAC-SHA1 routines, and that creating another ciphersuite for PIB-
HMAC (which does not exist in the base BSP specification) would
impose very little additional code. Partial support for the BSP's
elements (at least the PIB's format and mutable canonicalization)
could be made mandatory in the Bundle Protocol along with support for
the PIB-HMAC ciphersuite with NULL keys while retaining as optional
all of the other components of the BSP (BAB, PCB, and other
ciphersuites). This path seems to be desirable in that it allows re-
use of existing code along with re-use of existing specifications,
but does not significantly burden lightweight implementations or
deployments unconcerned with overlay-layer security. This approach
is followed in Section 3 within this document.
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Our previous proposal to add checksumming to bundles defined a new
block type for carrying error-detecting checksums computed over the
bundle payload. This was motivated by the fact that some deployments
on limited hardware within closed networks will not desire to run
heavyweight security protocols, nor include the full BSP and its
mandatory ciphersuites within their code footprints. Another
motivation was the desire to avoid creating a false sense of security
by using a mechanism labelled the Bundle _Security_ Protocol with
either a ciphersuite or NULL key that provides absolutely no security
services. For instance, if an implementation allowed this to be
configured using the same mechanisms or policy directive
configuration files, formats, etc. that are normally used to
configure BSP mechanisms providing real security, then a
misconfiguration or misunderstanding could have a negative security
impact to an operational system.
In order to allay these concerns, it was decided to define simple
error-detection ciphersuites with the string "INSECURE" in their
mnemonics and draw a line as to which portions of the BSP security
framework become mandatory and which remain optional. This allows
implementation of error-detection capabilities either with or without
the majority of the BSP, and with reduced potential for misleading
users with regards to security.
Implementations that provide both the full BSP and simple error-
detection ciphersuites SHOULD subjectively ensure that their
configuration by users is sufficiently dissimilar from the normal BSP
configuration. Implementations MUST NOT implement the "INSECURE"
ciphersuites in such a way that leads to their being construed as
security mechanisms in logging output or configuration directives.
3. INSECURE Ciphersuites
Any PIB ciphersuite providing only integrity checking for error-
detection and using published (or "null") keys MUST contain the
string "INSECURE" in its mnemonic. PIBs that use these ciphersuites
are otherwise indistinguishable from PIBs used to implement security
services. PIB-HMAC is keyed, and so does not use "INSECURE" in its
name. When used with a secret key, PIB-HMAC is useful for security,
although this is not the case when it is used with a NULL key, we
assume that the presence of a NULL key in the configuration
significantly alerts users to the fact that it is not providing
security.
To provide the desired functionality, three new ciphersuites are
defined in this document (PIB-HMAC used with either real or NULL
keys, PIB-INSECURE-MD5, and PIB-INSECURE-CRC32). The motivations
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behind defining all three of these ciphersuites are outlined below in
the more detailed description of each ciphersuite. All of the
ciphersuites defined here use the mutable canonicalization algorithm
that is defined in the BSP and compute their checksums over the
canonical forms of bundles. If error-detection support is considered
essential for use of the bundle protocol, this means that the minimal
BSP elements that all Bundle Protocol implementations MUST support
include:
1. Mutable Canonicalization - fully described in Appendix A.1 of
this document.
2. PIB wire-format - fully described in Appendix A.2 of this
document.
3. PIB-HMAC ciphersuite with NULL key definition, PIB-INSECURE-MD5
and PIB-INSECURE-CRC32 ciphersuite
The new ciphersuites are identified by the following ciphersuite IDs
within the abstract security block:
o 0x04 - PIB-HMAC
o 0x05 - PIB-INSECURE-MD5
o 0x06 - PIB-INSECURE-CRC32
PIB-HMAC is defined to use the same HMAC-SHA1 construction as the
BSP's BAB-HMAC ciphersuite, and can thus leverage existing code.
Three ciphersuite parameters are needed, all of which are SDNVs. The
first SDNV is a key identifier. A zero value in the key identifier
field of a PIB using the PIB-HMAC ciphersuite indicates that the
algorithm is keyed with the special NULL key. The NULL key used here
is defined to be 0xc37b 7e64 9258 4340 bed1 2207 8089 4115 5068 f738,
the same fixed NULL key used with LTP's NULL ciphersuite. The later
two EIDs are offsets describing the protected bits of the bundle,
identical to the offset and length parameters describe in the BSP's
PIB-RSA-SHA256 ciphersuite. The first identifies the first covered
octet and the second identifies the last covered octet.
PIB-HMAC creates a ten-octet security result and should provide
adequate error-detection capabilities for large bundles of at least
several gigabytes in size. Its advantage lies in that the NULL-keyed
version can be implemented with minor additions to existing BSP
codebases, and support for HMAC-SHA1 is known to only require around
200 lines of portable C code for implementations that do not already
contain BSP support.
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The existence of the PIB-INSECURE-MD5 ciphersuite is motivated by the
fact that an MD5 checksum can be computed on the order of twice as
fast as a SHA1 checksum over the same data, as demonstrated by
benchmarking activities [RFC1810], yet still yields robust error-
detection over fairly large inputs. This may be desirable in
environments that have only limited computational resources to expend
on bundle generation or processing. For instance, the authors have
implemented generation of bundles of up to several hundred megabytes
in size, onboard an imaging satellite solid-state data recorder using
only a 200 MHz processor. The PIB-INSECURE-MD5 parameters consist of
two SDNVs, an offset and length, that convey the covered portion of
the bundle, in an identical way to the corresponding PIB-HMAC and
PIB-RSA-SHA256 parameters.
The security result included with the PIB-INSECURE-MD5 ciphersuite is
a full 16-octet MD5 output. The longer security result than PIB-HMAC
may provide better error-detection for very large bundles, in
addition to being faster to compute. For small bundles, the lack of
the HMAC construction's second application of the hash function also
improves efficiency in PIB-INSECURE-MD5 compared to PIB-HMAC.
Implementations of MD5 are known to require only around 200 lines of
portable C code and are widely available as open-source and within
the MD5 RFC [RFC1321].
The PIB-INSECURE-CRC32 ciphersuite is intended for small bundles, and
SHOULD only be used on bundles whose payload length is less than
65535 bits, because its protection weakens for longer payloads due to
the increased risk of collisions [Koopman]. The parameters included
with this ciphersuite are identical to those used with PIB-INSECURE-
MD5. The security result is computed using the CRC-32c algorithm,
identically to that defined for use with SCTP [RFC3309]. The
security result is always a 4-octet quantity when PIB-INSECURE-CRC32
is used. The advantage of the CRC used as a checksum in this
ciphersuite is that it implies a lower header overhead to in-flight
or in-memory bundles with small payloads, in comparison to the length
of the PIB-HMAC and PIB-INSECURE-MD5 security results. This may be
highly desirable in environments where small messaging bundles are
normal and only bandwidth-limited links are available. The CRC-32c
algorithm is known to be implementable in only a few dozen lines of
portable C code.
3.1. Generation and Processing Rules
Since the INSECURE ciphersuites and NULL-keyed PIB-HMAC use the same
block type code and format as the more secure uses of PIB, they
inherit the existing generation and processing rules of the PIB.
This is good from a security standpoint in two respects:
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1. The existing PIB processing rules consider interaction with any
BABs that might be added to a bundle and prevent interactions
that would cause wrongful failure of MACs, signatures, and
checksums.
2. The PIB and PCB processing rules remove the possibility of a MAC,
signature, or checksum revealing information about the private
contents of the PCB via the ordering of the applied security/
error-detection transforms. This is discussed within the PCB-
RSA-AES128-PAYLOAD-PIB ciphersuite definition in the BSP
specification.
Although the BSP was intended as an optional suite of extensions to
the Bundle Protocol, and only needed in cases where certain security
services are desired at the bundle layer, a subset of its components
is always needed to implement the PIB-based error-detection
mechanism. As (1) providing error detection at some place in the
stack is needed by applications that require reliable delivery of
payload content, (2) many conceivable applications require such
delivery, and (3) the Bundle Protocol is a proposed new stack "waist
in the hourglass", the authors believe that error detection should be
supported in common implementations of the Bundle Protocol.
Supporting error-free delivery does not require mandatory
implementation of the full BSP and accompanying security
ciphersuites, but only requires the PIB block format, and the mutable
canonicalization rules. These two portions of the BSP are fully
described in the BSP specification [I-D.irtf-dtnrg-bundle-security]
with some commentary regarding their use for error-detection in
Appendix A of this document.
When non-error-tolerant applications request custody transfer, the
generated bundles MUST use an unkeyed or NULL-keyed PIB ciphersuite
defined in this document between the source and destination bundle
agents. Use of ciphersuites with private keys shared only by end-
hosts cannot assist in reliable hop-by-hop delivery, increasing the
time to error detection at the end nodes and the time required for
resends. For other bundles, not requiring custody transfer, an
unkeyed or NULL-keyed PIB ciphersuite SHOULD be used.
Checking of unkeyed or NULL-keyed PIBs at intermediate bundle agents
SHOULD be performed, when possible, and an agent which fails to match
the PIB security result within a bundle SHOULD immediately discard
the bundle. This limits the wasted resources involved in propagating
data now known to be errored. A future version of the Bundle
Protocol might include a bundle processing flag that signals that
errored-delivery is acceptable to a receiving application. However,
the current version does not define such a flag. A future version of
the Bundle Protocol specification might also define an administrative
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record that signals when a bundle has been dropped due to a
corruption event detected via an unkeyed or NULL-keyed PIB check;
that has not been defined in the current Bundle Protocol. The DTNRG
has not achieved consensus on either of these possibilities at time
of writing.
4. Performance Considerations
The normal method for handling error detection with security is to
cover the encrypted payload with an outer error-detecting checksum
wrapper. Use of an end-to-end private key without a separate end-to-
end outer error-detecting checksum prevents determination of the
bundle's fidelity by any in-path forwarding nodes lacking that
private key. This discourages interoperability between parties that
do not share keys, and consumes more network resources in relaying an
errored bundle to the receiving destination end node, and in
resending the bundle once the error is finally detected on
decryption. Custody transfer guaranteeing error-free receipt at
intermediate nodes is not possible with private keys in the mechanism
outlined above. When private keys are in use, errors introduced into
bundles can only be detected at the decoding endpoint, rather than at
intermediate nodes, meaning that resends across the entire network
are requested far later when security is in use. This means that
applications using unencrypted traffic can be expected to outperform
applications using private keys in DTN networks, thanks to their
ability to detect errors earlier, their smaller resend control loops
to get replacement bundles, and adding the capability for content
verification to the use of custody transfer.
Again, an outer error-detecting checksum around the encrypted or
unencrypted bundle prevents these problems, and allows custody
transfer to be meaningful for encrypted traffic.
5. Security Considerations
This document has attempted to assuage any security concerns that
would result from applying non-security-providing algorithms within a
mechanism intended for security. This is accomplished through
semantic overloading of the PIB, reusing its structure to hold a
simple checksum when it is not intended to provide security services.
The potential leakage of information if checksums are not covered by
some BSP confidentiality transform that is applied later in the
transmission path is eliminated by the fact that the existing PIB
block type code is used, and the BSP itself already contains rules
for ensuring that confidentiality transforms applied by the PCB
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protect the security result fields within PIB instances.
This design decision to reuse a security block for error-detection
may seem bizarre to both security and networking experts. However,
this decision was necessitated by the late addition of checksum
support to the Bundle Protocol. By the time interest in this subject
arose within the DTNRG, the Bundle Protocol itself was in final
review phases and had been implemented multiple times. At the time
of this writing, the group did not have consensus that block
validation and payload error detection even belonged in the Bundle
Protocol itself. The Bundle Security Protocol was also no longer
malleable enough to ensure compatibility with checksum support, as it
had obtained a level of relative stability in its specification and
there were existing implementation efforts based on these which could
have required modification in order to not pass checksums carried in
a non-BSP block as unprotected after performing a confidentiality
transform of the payload.
In the authors' opinions, ideally, the error-detection functions
would be implemented within the basic networking portions of the
Bundle Protocol, and not as a subset of the security framework.
However, the existing Bundle Protocol design was too well-established
for the current definition of the Bundle Protocol to be affected. At
this time, the Bundle Protocol has only been proposed as Experimental
with some disclaimers. It is felt by some that future revisions of
the Bundle Protocol need to provide mechanisms ensuring error
detection for reliable delivery. In order to limit overhead, shorter
checksums, e.g. CRC-16, could be used for small blocks, with longer
checksums, e.g. MD5, reserved for large payload blocks. This would
allow checksums to cover and provide confident processing of even
blocks with mutable fields, and retain efficient updating in-network
as a mutable field changes, without the recomputation also covering
large unchanging payload blocks. This might also constrain the
damage caused by errors to the functions provided by an individual
block, rather than affecting the whole bundle and causing the whole
bundle to be discarded, although the overall value of this is
currently unknown.
The need to conserve limited network resources by detecting and
avoiding further propagating errored bundles in-transit means that
Bundle Agents SHOULD always validate checksums of in-flight bundles,
even if the Agents are not the ultimate destination. This opens the
door for a potential denial-of-service attack on DTN Bundle Agents by
forcing them to expend computational cycles on bundles with large
payloads. In this case, the attacker would also have to send these
bundles over some link towards the target Bundle Agent, which will
often be more constrained in bandwidth or availability than the
Bundle Agent is in compuational cycles, so this threat may be
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unrealistic, or better combatted through access-control on links. If
this threat does turn out to be realistic in some set of
circumstances, intermediary validation of PIBs was intentionally left
as a SHOULD-level activity rather than a MUST, and could be
dynamically disabled at some threshold of CPU use.
Using the same protocol mechanism to provide (1) error-detection
without security claims, (2) error-detection using a security
protocol insecurely-keyed with a known NULL key, and (3) actual
security protection using the same protocol but with secret keys, any
of which can defined and used in the same "Payload Integrity Block",
is confusing at best, and not a good clean-sheet approach to helping
ensure secure configurations, interoperable implementations, or
efficient handling of errored bundles.
Use of a NULL key is inferior to separately handling the security
concerns of sender-authentication and integrity-protection from that
of error-checking as it opens the door to private keys that prevent
standalone error-detection, and should be discouraged. Also, the
NULL key cannot provide error-detection needed for the mutable parts
of the bundle. Providing any error detection for the mutable parts
of the bundle has not been done here, and reliance on the fidelity of
mutable payloads should be avoided for this reason.
6. IANA Considerations
This document has no considerations for IANA.
7. Acknowledgements
Some of the work on this document was performed at NASA's Glenn
Research Center under funding from the Earth Science Technology
Office (ESTO) and the Space Communications Architecture Working Group
(SCAWG).
Discussion in the DTNRG and particular suggestions from
(alphabetically) Mike Demmer, Kevin Fall, Stephen Farrell, Darren
Long, Peter Lovell, and Susan Symington guided the genesis of this
document and were crucial to adding limited error-detection
capabilities to the Bundle Protocol within the existing pre-
established security framework.
8. References
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8.1. Normative References
[I-D.irtf-dtnrg-bundle-security]
Symington, S., Farrell, S., Weiss, H., and P. Lovell,
"Bundle Security Protocol Specification",
draft-irtf-dtnrg-bundle-security-05 (work in progress),
February 2008.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC 3309,
September 2002.
[RFC5050] Scott, K. and S. Burleigh, "Bundle Protocol
Specification", RFC 5050, November 2007.
8.2. Informative References
[I-D.farrell-dtnrg-km]
Farrell, S., "DTN Key Management Requirements",
draft-farrell-dtnrg-km-00 (work in progress), June 2007.
[I-D.irtf-dtnrg-ltp-extensions]
Farrell, S., Ramadas, M., and S. Burleigh, "Licklider
Transmission Protocol - Extensions",
draft-irtf-dtnrg-ltp-extensions-06 (work in progress),
October 2007.
[I-D.irtf-dtnrg-sec-overview]
Farrell, S., Symington, S., Weiss, H., and P. Lovell,
"Delay-Tolerant Networking Security Overview",
draft-irtf-dtnrg-sec-overview-04 (work in progress),
February 2008.
[I-D.irtf-dtnrg-tcp-clayer]
Demmer, M. and J. Ott, "Delay Tolerant Networking TCP
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Internet-Draft Bundle Checksum Ciphersuites March 2008
Convergence Layer Protocol",
draft-irtf-dtnrg-tcp-clayer-01 (work in progress),
February 2008.
[Koopman] Koopman, P., "32-bit cyclic redundancy codes for Internet
applications", Proceedings of the International Conference
on Dependable Systems and Networks (DSN), pp. 459-
468 2002, June 2002.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662,
July 1994.
[RFC1810] Touch, J., "Report on MD5 Performance", RFC 1810,
June 1995.
[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.
[SGHP98] Stone, J., Greenwald, M., Hughes, J., and C. Partridge,
"Performance of checksums and CRCs over real data", IEEE
Transactions on Networks vol. 6 issue 5, pp. 529-543,
October 1998.
[SP00] Stone, J. and C. Partridge, "When the CRC and TCP Checksum
Disagree", Proceedings of ACM SIGCOMM 2000,
September 2000.
[SRC84] Saltzer, J., Reed, D., and D. Clark, "End-to-end Arguments
in System Design", ACM Transactions on Computer Systems 2
(4), November 1984.
Appendix A. Mandatory BSP Elements Needed to Implement Error-Detection
This document makes some BSP components mandatory to Bundle Protocol
implementations, in that while their use is optional they must be
supported for interoperability reasons. Previously, the BSP was
previously entirely optional. The appendix discusses these elements
in greater detail, and highlights some further drawbacks of basing
error detection and protocol reliability upon them.
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A.1. Mutable Canonicalization
While impressively named, the mutable canonicalization procedure
should actually be quite simple to understand. The requirement for
mutable canonicalization stems from the Bundle Protocol's forwarding
design that allows several "mutable" fields (e.g. the "dictionary",
custodian, and some flags and length fields), to change in-transit at
intermediate nodes. In order for the checksum, MAC, or signature
computed and placed in the security result of a sent PIB to match the
result computed over the received bundle, the sender and receiver
need to leave mutable fields out of these computations. The format
of a bundle that is input to the PIB algorithms thus differs from its
wire-format, and is called its "mutable canonicalization".
Using mutable canonicalization implies either using an incrementally-
updatable checksum algorithm and feeding many small pieces of data to
it, or entirely rewriting a bundle block-by-block based on mutable
canonicalization rules before feeding it to the checksum function.
(The mutable fields still require protection against errors; a hop-
by-hop checksum over only the mutable fields could be used to provide
this. Hop-by-hop checksum coverage could be provided by a
convergence layer or BAB, but this would likely cover the entire
bundle or fragment.)
Several problems are known to plague mutable canonicalization:
1. [EDITOR'S NOTE: This concern may be easily fixed in updates to
the BSP document.] The Bundle Protocol specification describes
the bundle processing control flags as a single variable-length
SDNV whose bits are sub-divided in-order by function into "SRR"
(Status Report Request), "COS" (Class of Service), and "General".
The BSP's mutable canonicalization description shows three
separate fields, with only slightly differing names, but in
totally opposite order: "Proc. Flags", "COS Flags", and "SRR
Flags", instead of one single SDNV, yet has text that describes
operating on these as a single 64-bit value and applying a fixed-
length mask to them. This is unclear at best, and feared to be
uninteroperable in implementations.
2. Many bits within the bundle processing control flags are masked
out (i.e. forced to zero within) the mutable canonicalization
format. This includes all of the reserved class of service (COS)
bits that are highly likely to be needed to overcome the
limitation of having only three defined priority levels in the
Bundle Specification (compare to DiffServ, CLNP's priority field,
Aeronautical Mobile Radio Service message priorities, or
mechanisms in other networking stacks that provide many more
bits). This means that these bits, and any other bundle
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processing control bits, will be unprotected by the end-to-end
checksum and may change in-transit, potentially causing mis-
treatment or mal-delivery of bundles.
3. The existing "bundle is a fragment" bit is unprotected in mutable
canonicalization. Errors in this bit itself can probably be
caught through other means, such as careful length and bound
checking in processing the rest of the bundle.
4. The entire mutable canonicalization procedure of parsing and re-
formatting bundles in order to perform a checksum validation is
significantly more complex than is typical in most existing
protocols that are designed to be capable of simply computing a
validation over a frame either without modifications [RFC1662],
with only a small fixed-length and position field masked
[RFC0791], or with only a simple fixed-size pseudoheader
[RFC0793]. The significant additional complexity of mutable
canonicalization prevents high performance in forwarding nodes
that follow the guideline of verifying unkeyed or NULL-keyed
PIBs.
A.2. PIB Format
PIBs follow the format of the abstract security block, defined in the
BSP specification [I-D.irtf-dtnrg-bundle-security], with a block type
code that identifies them as PIBs. Some of the processing rules for
PIBs that make the PIB less than ideal for error-detection purposes
include:
1. If a PCB is placed into a bundle that already has a PSB, then
another PCB is created that hides the PIB. This means that for
end-to-end error-detection PIBs, any in-network security proxies
that add PCB blocks also prevent the checksum in the PIB from
being verifiable before the PCB's security destination recovers
the cleartext PIB. If the PCB security destination is never
reached, the bundle cannot be checked for errors. Errored
bundles will consume resources between these two security
gateways, since the errors cannot be detected and the bundles
discarded en route. The RSA signature of such an errored bundle
will only fail at the security destination, and the bundle will
only be discarded at that end point, but there may be significant
resources expended in delivering the useless bundle to that
point.
2. A previously-generated PIB's security result cannot be retained
outside a PCB in the clear, because an observer could correlate
the value to some known probable plaintext payload value. It
might be better to reverse the order of operations and always
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generate rewritten PIB ciphersuite checksums after generating
PCBs that encrypt the payload, so that the PIB security result
covers the PCB's encrypted form of the payload rather than the
unencrypted form, and uses the same security destination as the
PCB. Upon reaching this security destination, another PIB
destined for the receiver, covering the payload revealed at the
security destination, could be generated. Requiring this would
allow detection of errored bundles between PCB security source
and PCB security destination, but would involve adding another
instruction to the PCB generation process within the BSP. This
assumes no errors are introduced during the decryption process of
the PCB, as such errors would go undetected. If bundles pass
through nested security domains, this could compound the error
rate.
There seems to be both benefits and drawbacks to any approach to PIB
and PCB interaction that does not involve layering multiple PIBs that
can be pushed and popped off of a bundle at various security sources
and destinations. Pushing and popping nested PSBs approximates the
outer checksum around inner security payload used successfully
elsewhere in networking. By some reasonable metrics, the BSP-
prescribed interaction that we have attempted to build on and fix
here may be among the least desirable of all known methods for error-
detection.
Authors' Addresses
Wesley M. Eddy
Verizon Federal Network Systems
NASA Glenn Research Center
21000 Brookpark Rd, MS 54-5
Cleveland, OH 44135
United States of America
Phone: +1-216-433-6682
Email: weddy@grc.nasa.gov
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
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Will Ivancic
NASA Glenn Research Center
21000 Brookpark Road
Cleveland, Ohio 44135
USA
Phone: +1-216-433-3494
Fax: +1-216-433-8705
Email: William.D.Ivancic@nasa.gov
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