RFC 4656
A One-way Active Measurement Protocol (OWAMP)
4. OWAMP-Test
This section describes OWAMP-Test protocol. It runs over UDP, using sender and receiver IP and port numbers negotiated during the Request-Session exchange.
As with OWAMP-Control, OWAMP-Test has three modes: unauthenticated, authenticated, and encrypted. All OWAMP-Test sessions that are spawned by an OWAMP-Control session inherit its mode. OWAMP-Control client, OWAMP-Control server, OWAMP-Test sender, and OWAMP-Test receiver can potentially all be different machines. (In a typical case, we expect that there will be only two machines.)4.1. Sender Behavior
4.1.1. Packet Timings
Send schedules based on slots, described previously, in conjunction with scheduled session start time, enable the sender and the receiver to compute the same exact packet sending schedule independently of each other. These sending schedules are independent for different OWAMP-Test sessions, even if they are governed by the same OWAMP- Control session. Consider any OWAMP-Test session. Once Start-Sessions exchange is complete, the sender is ready to start sending packets. Under normal OWAMP use circumstances, the time to send the first packet is in the near future (perhaps a fraction of a second away). The sender SHOULD send packets as close as possible to their scheduled time, with the following exception: if the scheduled time to send is in the past, and is separated from the present by more than Timeout time, the sender MUST NOT send the packet. (Indeed, such a packet would be considered lost by the receiver anyway.) The sender MUST keep track of which packets it does not send. It will use this to tell the receiver what packets were not sent by setting Skip Ranges in the Stop-Sessions message from the sender to the receiver upon completion of the test. The Skip Ranges are also sent to a Fetch-Client as part of the session data results. These holes in the sending schedule can happen if a time in the past was specified in the Request-Session command, or if the Start-Sessions exchange took unexpectedly long, or if the sender could not start serving the OWAMP-Test session on time due to internal scheduling problems of the OS. Packets that are in the past but are separated from the present by less than Timeout value SHOULD be sent as quickly as possible. With normal test rates and timeout values, the number of packets in such a burst is limited. Nevertheless, hosts SHOULD NOT intentionally schedule sessions so that such bursts of packets occur. Regardless of any scheduling delays, each packet that is actually sent MUST have the best possible approximation of its real time of departure as its timestamp (in the packet).
4.1.2. OWAMP-Test Packet Format and Content
The sender sends the receiver a stream of packets with the schedule specified in the Request-Session command. The sender SHOULD set the TTL in IPv4 (or Hop Limit in IPv6) in the UDP packet to 255. The format of the body of a UDP packet in the stream depends on the mode being used. For unauthenticated mode: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Error Estimate | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | . . . Packet Padding . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For authenticated and encrypted modes:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| MBZ (12 octets) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Estimate | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| MBZ (6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| HMAC (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Packet Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of the timestamp is the same as in [RFC1305] and is as
follows: the first 32 bits represent the unsigned integer number of
seconds elapsed since 0h on 1 January 1900; the next 32 bits
represent the fractional part of a second that has elapsed since
then.
So, Timestamp is represented as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integer part of seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fractional part of seconds |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Estimate specifies the estimate of the error and
synchronization. It has the following format:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|Z| Scale | Multiplier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first bit, S, SHOULD be set if the party generating the timestamp
has a clock that is synchronized to UTC using an external source
(e.g., the bit should be set if GPS hardware is used and it indicates
that it has acquired current position and time or if NTP is used and
it indicates that it has synchronized to an external source, which
includes stratum 0 source, etc.). If there is no notion of external
synchronization for the time source, the bit SHOULD NOT be set. The
next bit has the same semantics as MBZ fields elsewhere: it MUST be
set to zero by the sender and ignored by everyone else. The next six
bits, Scale, form an unsigned integer; Multiplier is an unsigned
integer as well. They are interpreted as follows: the error estimate
is equal to Multiplier*2^(-32)*2^Scale (in seconds). (Notation
clarification: 2^Scale is two to the power of Scale.) Multiplier
MUST NOT be set to zero. If Multiplier is zero, the packet SHOULD be
considered corrupt and discarded.
Sequence numbers start with zero and are incremented by one for each
subsequent packet.
The minimum data segment length is, therefore, 14 octets in
unauthenticated mode, and 48 octets in both authenticated mode and
encrypted modes.
The OWAMP-Test packet layout is the same in authenticated and
encrypted modes. The encryption and authentication operations are,
however, different. The difference is that in encrypted mode both
the sequence number and the timestamp are protected to provide
maximum data confidentiality and integrity protection, whereas in
authenticated mode the sequence number is protected while the
timestamp is sent in clear text. Sending the timestamp in clear text
in authenticated mode allows one to reduce the time between when a
timestamp is obtained by a sender and when the packet is shipped out.
In encrypted mode, the sender has to fetch the timestamp, encrypt it,
and send it; in authenticated mode, the middle step is removed,
potentially improving accuracy (the sequence number can be encrypted
and authenticated before the timestamp is fetched).
In authenticated mode, the first block (16 octets) of each packet is
encrypted using AES Electronic Cookbook (ECB) mode.
Similarly to each OWAMP-Control session, each OWAMP-Test session has two keys: an AES Session-key and an HMAC Session-key. However, there is a difference in how the keys are obtained: in the case of OWAMP- Control, the keys are generated by the client and communicated (as part of the Token) during connection setup as part of Set-Up-Response message; in the case of OWAMP-Test, described here, the keys are derived from the OWAMP-Control keys and the SID. The OWAMP-Test AES Session-key is obtained as follows: the OWAMP- Control AES Session-key (the same AES Session-key as is used for the corresponding OWAMP-Control session, where it is used in a different chaining mode) is encrypted, using AES, with the 16-octet session identifier (SID) as the key; this is a single-block ECB encryption; its result is the OWAMP-Test AES Session-key to use in encrypting (and decrypting) the packets of the particular OWAMP-Test session. Note that all of OWAMP-Test AES Session-key, OWAMP-Control AES Session-key, and the SID are comprised of 16 octets. The OWAMP-Test HMAC Session-key is obtained as follows: the OWAMP- Control HMAC Session-key (the same HMAC Session-key as is used for the corresponding OWAMP-Control session) is encrypted, using AES, with the 16-octet session identifier (SID) as the key; this is a two-block CBC encryption, always performed with IV=0; its result is the OWAMP-Test HMAC Session-key to use in authenticating the packets of the particular OWAMP-Test session. Note that all of OWAMP-Test HMAC Session-key and OWAMP-Control HMAC Session-key are comprised of 32 octets, while the SID is 16 octets. ECB mode used for encrypting the first block of OWAMP-Test packets in authenticated mode does not involve any actual chaining; this way, lost, duplicated, or reordered packets do not cause problems with deciphering any packet in an OWAMP-Test session. In encrypted mode, the first two blocks (32 octets) are encrypted using AES CBC mode. The AES Session-key to use is obtained in the same way as the key for authenticated mode. Each OWAMP-Test packet is encrypted as a separate stream, with just one chaining operation; chaining does not span multiple packets so that lost, duplicated, or reordered packets do not cause problems. The initialization vector for the CBC encryption is a value with all bits equal to zero. Implementation note: Naturally, the key schedule for each OWAMP-Test session MAY be set up only once per session, not once per packet.
HMAC in OWAMP-Test only covers the part of the packet that is also encrypted. So, in authenticated mode, HMAC covers the first block (16 octets); in encrypted mode, HMAC covers two first blocks (32 octets). In OWAMP-Test HMAC is not encrypted (note that this is different from OWAMP-Control, where encryption in stream mode is used, so everything including the HMAC blocks ends up being encrypted). In unauthenticated mode, no encryption or authentication is applied. Packet Padding in OWAMP-Test SHOULD be pseudo-random (it MUST be generated independently of any other pseudo-random numbers mentioned in this document). However, implementations MUST provide a configuration parameter, an option, or a different means of making Packet Padding consist of all zeros. The time elapsed between packets is computed according to the slot schedule as mentioned in Request-Session command description. At that point, we skipped over the issue of computing exponentially distributed pseudo-random numbers in a reproducible fashion. It is discussed later in a separate section.4.2. Receiver Behavior
The receiver knows when the sender will send packets. The following parameter is defined: Timeout (from Request-Session). Packets that are delayed by more than Timeout are considered lost (or "as good as lost"). Note that there is never an actual assurance of loss by the network: a "lost" packet might still be delivered at any time. The original specification for IPv4 required that packets be delivered within TTL seconds or never (with TTL having a maximum value of 255). To the best of the authors' knowledge, this requirement was never actually implemented (and, of course, only a complete and universal implementation would ensure that packets do not travel for longer than TTL seconds). In fact, in IPv6, the name of this field has actually been changed to Hop Limit. Further, IPv4 specification makes no claims about the time it takes the packet to traverse the last link of the path. The choice of a reasonable value of Timeout is a problem faced by a user of OWAMP protocol, not by an implementor. A value such as two minutes is very safe. Note that certain applications (such as interactive "one-way ping" might wish to obtain the data faster than that. As packets are received, + timestamp the received packet;
+ in authenticated or encrypted mode, decrypt and authenticate as
necessary (packets for which authentication fails MUST be
discarded); and
+ store the packet sequence number, send time, receive time, and the
TTL for IPv4 (or Hop Limit for IPv6) from the packet IP header for
the results to be transferred.
Packets not received within the Timeout are considered lost. They
are recorded with their true sequence number, presumed send time,
receive time value with all bits being zero, and a TTL (or Hop Limit)
of 255.
Implementations SHOULD fetch the TTL/Hop Limit value from the IP
header of the packet. If an implementation does not fetch the actual
TTL value (the only good reason not to do so is an inability to
access the TTL field of arriving packets), it MUST record the TTL
value as 255.
Packets that are actually received are recorded in the order of
arrival. Lost packet records serve as indications of the send times
of lost packets. They SHOULD be placed either at the point where the
receiver learns about the loss or at any later point; in particular,
one MAY place all the records that correspond to lost packets at the
very end.
Packets that have send time in the future MUST be recorded normally,
without changing their send timestamp, unless they have to be
discarded. (Send timestamps in the future would normally indicate
clocks that differ by more than the delay. Some data -- such as
jitter -- can be extracted even without knowledge of time difference.
For other kinds of data, the adjustment is best handled by the data
consumer on the basis of the complete information in a measurement
session, as well as, possibly, external data.)
Packets with a sequence number that was already observed (duplicate
packets) MUST be recorded normally. (Duplicate packets are sometimes
introduced by IP networks. The protocol has to be able to measure
duplication.)
If any of the following is true, the packet MUST be discarded:
+ Send timestamp is more than Timeout in the past or in the future.
+ Send timestamp differs by more than Timeout from the time when the
packet should have been sent according to its sequence number.
+ In authenticated or encrypted mode, HMAC verification fails.
5. Computing Exponentially Distributed Pseudo-Random Numbers
Here we describe the way exponential random quantities used in the protocol are generated. While there is a fair number of algorithms for generating exponential random variables, most of them rely on having logarithmic function as a primitive, resulting in potentially different values, depending on the particular implementation of the math library. We use algorithm 3.4.1.S from [KNUTH], which is free of the above-mentioned problem, and which guarantees the same output on any implementation. The algorithm belongs to the ziggurat family developed in the 1970s by G. Marsaglia, M. Sibuya, and J. H. Ahrens [ZIGG]. It replaces the use of logarithmic function by clever bit manipulation, still producing the exponential variates on output.5.1. High-Level Description of the Algorithm
For ease of exposition, the algorithm is first described with all arithmetic operations being interpreted in their natural sense. Later, exact details on data types, arithmetic, and generation of the uniform random variates used by the algorithm are given. It is an almost verbatim quotation from [KNUTH], p.133. Algorithm S: Given a real positive number "mu", produce an exponential random variate with mean "mu". First, the constants Q[k] = (ln2)/(1!) + (ln2)^2/(2!) + ... + (ln2)^k/(k!), 1 <= k <= 11 are computed in advance. The exact values which MUST be used by all implementations are given in the next section. This is necessary to ensure that exactly the same pseudo-random sequences are produced by all implementations. S1. [Get U and shift.] Generate a 32-bit uniform random binary fraction U = (.b0 b1 b2 ... b31) [note the binary point] Locate the first zero bit b_j and shift off the leading (j+1) bits, setting U <- (.b_{j+1} ... b31) Note: In the rare case that the zero has not been found, it is prescribed that the algorithm return (mu*32*ln2). S2. [Immediate acceptance?] If U < ln2, set X <- mu*(j*ln2 + U) and terminate the algorithm. (Note that Q[1] = ln2.)
S3. [Minimize.] Find the least k >= 2 such that U < Q[k]. Generate k new uniform random binary fractions U1,...,Uk and set V <- min(U1,...,Uk). S4. [Deliver the answer.] Set X <- mu*(j + V)*ln2.5.2. Data Types, Representation, and Arithmetic
The high-level algorithm operates on real numbers, typically represented as floating point numbers. This specification prescribes that unsigned 64-bit integers be used instead. u_int64_t integers are interpreted as real numbers by placing the decimal point after the first 32 bits. In other words, conceptually, the interpretation is given by the following map: u_int64_t u; u |--> (double)u / (2**32) The algorithm produces a sequence of such u_int64_t integers that, for any given value of SID, is guaranteed to be the same on any implementation. We specify that the u_int64_t representations of the first 11 values of the Q array in the high-level algorithm MUST be as follows: #1 0xB17217F8, #2 0xEEF193F7, #3 0xFD271862, #4 0xFF9D6DD0, #5 0xFFF4CFD0, #6 0xFFFEE819, #7 0xFFFFE7FF, #8 0xFFFFFE2B, #9 0xFFFFFFE0, #10 0xFFFFFFFE, #11 0xFFFFFFFF For example, Q[1] = ln2 is indeed approximated by 0xB17217F8/(2**32) = 0.693147180601954; for j > 11, Q[j] is 0xFFFFFFFF. Small integer j in the high-level algorithm is represented as u_int64_t value j * (2**32). Operation of addition is done as usual on u_int64_t numbers; however, the operation of multiplication in the high-level algorithm should be replaced by
(u, v) |---> (u * v) >> 32.
Implementations MUST compute the product (u * v) exactly. For
example, a fragment of unsigned 128-bit arithmetic can be implemented
for this purpose (see the sample implementation in Appendix A).
5.3. Uniform Random Quantities
The procedure for obtaining a sequence of 32-bit random numbers (such
as U in algorithm S) relies on using AES encryption in counter mode.
To describe the exact working of the algorithm, we introduce two
primitives from Rijndael. Their prototypes and specification are
given below, and they are assumed to be provided by the supporting
Rijndael implementation, such as [RIJN].
+ A function that initializes a Rijndael key with bytes from seed
(the SID will be used as the seed):
void KeyInit(unsigned char seed[16]);
+ A function that encrypts the 16-octet block inblock with the
specified key, returning a 16-octet encrypted block. Here,
keyInstance is an opaque type used to represent Rijndael keys:
void BlockEncrypt(keyInstance key, unsigned char inblock[16]);
Algorithm Unif: given a 16-octet quantity seed, produce a sequence of
unsigned 32-bit pseudo-random uniformly distributed integers. In
OWAMP, the SID (session ID) from Control protocol plays the role of
seed.
U1. [Initialize Rijndael key] key <- KeyInit(seed) [Initialize an
unsigned 16-octet (network byte order) counter] c <- 0
U2. [Need more random bytes?] Set i <- c mod 4. If (i == 0) set s
<- BlockEncrypt(key, c)
U3. [Increment the counter as unsigned 16-octet quantity] c <- c + 1
U4. [Do output] Output the i_th quartet of octets from s starting
from high-order octets, converted to native byte order and
represented as OWPNum64 value (as in 3.b).
U5. [Loop] Go to step U2.
6. Security Considerations
6.1. Introduction
The goal of authenticated mode is to let one passphrase-protect the service provided by a particular OWAMP-Control server. One can imagine a variety of circumstances where this could be useful. Authenticated mode is designed to prohibit theft of service. An additional design objective of the authenticated mode was to make it impossible for an attacker who cannot read traffic between OWAMP- Test sender and receiver to tamper with test results in a fashion that affects the measurements, but not other traffic. The goal of encrypted mode is quite different: to make it hard for a party in the middle of the network to make results look "better" than they should be. This is especially true if one of client and server does not coincide with either sender or receiver. Encryption of OWAMP-Control using AES CBC mode with blocks of HMAC after each message aims to achieve two goals: (i) to provide secrecy of exchange, and (ii) to provide authentication of each message.6.2. Preventing Third-Party Denial of Service
OWAMP-Test sessions directed at an unsuspecting party could be used for denial of service (DoS) attacks. In unauthenticated mode, servers SHOULD limit receivers to hosts they control or to the OWAMP- Control client. Unless otherwise configured, the default behavior of servers MUST be to decline requests where the Receiver Address field is not equal to the address that the control connection was initiated from or an address of the server (or an address of a host it controls). Given the TCP handshake procedure and sequence numbers in the control connection, this ensures that the hosts that make such requests are actually those hosts themselves, or at least on the path towards them. If either this test or the handshake procedure were omitted, it would become possible for attackers anywhere in the Internet to request that large amounts of test packets be directed against victim nodes somewhere else. In any case, OWAMP-Test packets with a given source address MUST only be sent from the node that has been assigned that address (i.e., address spoofing is not permitted).
6.3. Covert Information Channels
OWAMP-Test sessions could be used as covert channels of information. Environments that are worried about covert channels should take this into consideration.6.4. Requirement to Include AES in Implementations
Notice that AES, in counter mode, is used for pseudo-random number generation, so implementation of AES MUST be included even in a server that only supports unauthenticated mode.6.5. Resource Use Limitations
An OWAMP server can consume resources of various kinds. The two most important kinds of resources are network capacity and memory (primary or secondary) for storing test results. Any implementation of OWAMP server MUST include technical mechanisms to limit the use of network capacity and memory. Mechanisms for managing the resources consumed by unauthenticated users and users authenticated with a KeyID and passphrase SHOULD be separate. The default configuration of an implementation MUST enable these mechanisms and set the resource use limits to conservatively low values. One way to design the resource limitation mechanisms is as follows: assign each session to a user class. User classes are partially ordered with "includes" relation, with one class ("all users") that is always present and that includes any other class. The assignment of a session to a user class can be based on the presence of authentication of the session, the KeyID, IP address range, time of day, and, perhaps, other factors. Each user class would have a limit for usage of network capacity (specified in units of bit/second) and memory for storing test results (specified in units of octets). Along with the limits for resource use, current use would be tracked by the server. When a session is requested by a user in a specific user class, the resources needed for this session are computed: the average network capacity use (based on the sending schedule) and the maximum memory use (based on the number of packets and number of octets each packet would need to be stored internally -- note that outgoing sessions would not require any memory use). These resource use numbers are added to the current resource use numbers for the given user class; if such addition would take the resource use outside of the limits for the given user class, the session is rejected. When resources are reclaimed, corresponding measures are subtracted from the current use. Network capacity is reclaimed as soon as the session ends. Memory is reclaimed when the data is
deleted. For unauthenticated sessions, memory consumed by an OWAMP- Test session SHOULD be reclaimed after the OWAMP-Control connection that initiated the session is closed (gracefully or otherwise). For authenticated sessions, the administrator who configures the service should be able to decide the exact policy, but useful policy mechanisms that MAY be implemented are the ability to automatically reclaim memory when the data is retrieved and the ability to reclaim memory after a certain configurable (based on user class) period of time passes after the OWAMP-Test session terminates.6.6. Use of Cryptographic Primitives in OWAMP
At an early stage in designing the protocol, we considered using Transport Layer Security (TLS) [RFC2246, RFC3546] and IPsec [RFC2401] as cryptographic security mechanisms for OWAMP; later, we also considered DTLS. The disadvantages of those are as follows (not an exhaustive list): Regarding TLS: + TLS could be used to secure TCP-based OWAMP-Control, but it would be difficult to use it to secure UDP-based OWAMP-Test: OWAMP-Test packets, if lost, are not resent, so packets have to be (optionally) encrypted and authenticated while retaining individual usability. Stream-based TLS cannot be easily used for this. + Dealing with streams, TLS does not authenticate individual messages (even in OWAMP-Control). The easiest way out would be to add some known-format padding to each message and to verify that the format of the padding is intact before using the message. The solution would thus lose some of its appeal ("just use TLS"). It would also be much more difficult to evaluate the security of this scheme with the various modes and options of TLS; it would almost certainly not be secure with all. The capacity of an attacker to replace parts of messages (namely, the end) with random garbage could have serious security implications and would need to be analyzed carefully. Suppose, for example, that a parameter that is used in some form to control the rate were replaced by random garbage; chances are that the result (an unsigned integer) would be quite large. + Dependent on the mode of use, one can end up with a requirement for certificates for all users and a PKI. Even if one is to accept that PKI is desirable, there just isn't a usable one today.
+ TLS requires a fairly large implementation. OpenSSL, for example,
is larger than our implementation of OWAMP as a whole. This can
matter for embedded implementations.
Regarding DTLS:
+ Duplication and, similarly, reordering are network phenomena that
OWAMP needs to be able to measure; yet anti-replay measures and
reordering protection of DTLS would prevent the duplicated and
reordered packets from reaching the relevant part of the OWAMP
code. One could, of course, modify DTLS so that these protections
are weakened or even specify examining the messages in a carefully
crafted sequence somewhere in between DTLS checks; but then, of
course, the advantage of using an existing protocol would not be
realized.
+ In authenticated mode, the timestamp is in the clear and is not
protected cryptographically in any way, while the rest of the
message has the same protection as in encrypted mode. This mode
allows one to trade off cryptographic protection against accuracy
of timestamps. For example, the APAN hardware implementation of
OWAMP [APAN] is capable of supporting authenticated mode. The
accuracy of these measurements is in the sub-microsecond range.
The errors in OWAMP measurements of Abilene [Abilene] (done using
a software implementation, in its encrypted mode) exceed 10us.
Users in different environments have different concerns, and some
might very well care about every last microsecond of accuracy. At
the same time, users in these same environments might care about
access control to the service. Authenticated mode permits them to
control access to the server yet to use unprotected timestamps,
perhaps generated by a hardware device.
Regarding IPsec:
+ What we now call authenticated mode would not be possible (in
IPsec you can't authenticate part of a packet).
+ The deployment paths of IPsec and OWAMP could be separate if OWAMP
does not depend on IPsec. After nine years of IPsec, only 0.05%
of traffic on an advanced backbone network, such as Abilene, uses
IPsec (for comparison purposes with encryption above layer 4, SSH
use is at 2-4% and HTTPS use is at 0.2-0.6%). It is desirable to
be able to deploy OWAMP on as large a number of different
platforms as possible.
+ The deployment problems of a protocol dependent on IPsec would be
especially acute in the case of lightweight embedded devices.
Ethernet switches, DSL "modems", and other such devices mostly do
not support IPsec.
+ The API for manipulating IPsec from an application is currently
poorly understood. Writing a program that needs to encrypt some
packets, to authenticate some packets, and to leave some open --
for the same destination -- would become more of an exercise in
IPsec than in IP measurement.
For the enumerated reasons, we decided to use a simple cryptographic
protocol (based on a block cipher in CBC mode) that is different from
TLS and IPsec.
6.7. Cryptographic Primitive Replacement
It might become necessary in the future to replace AES, or the way it
is used in OWAMP, with a new cryptographic primitive, or to make
other security-related changes to the protocol. OWAMP provides a
well-defined point of extensibility: the Modes word in the server
greeting and the Mode response in the Set-Up-Response message. For
example, if a simple replacement of AES with a different block cipher
with a 128-bit block is needed, this could be accomplished as
follows: take two bits from the reserved (MBZ) part of the Modes word
of the server greeting; use one of these bits to indicate encrypted
mode with the new cipher and another one to indicate authenticated
mode with the new cipher. (Bit consumption could, in fact, be
reduced from two to one, if the client is allowed to return a mode
selection with more than a single bit set: one could designate a
single bit to mean that the new cipher is supported (in the case of
the server) or selected (in the case of the client) and continue to
use already allocated bits for authenticated and encrypted modes;
this optimization is unimportant conceptually, but it could be useful
in practice to make the best use of bits.) Then, if the new cipher
is negotiated, all subsequent operations simply use it instead of
AES. Note that the normal transition sequence would be used in such
a case: implementations would probably first start supporting and
preferring the new cipher, and then drop support for the old cipher
(presumably no longer considered secure).
If the need arises to make more extensive changes (perhaps to replace AES with a 256-bit-block cipher), this would be more difficult and would require changing the layout of the messages. However, the change can still be conducted within the framework of OWAMP extensibility using the Modes/Mode words. The semantics of the new bits (or single bit, if the optimization described above is used) would include the change to message layout as well as the change in the cryptographic primitive. Each of the bits in the Modes word can be used for an independent extension. The extensions signaled by various bits are orthogonal; for example, one bit might be allocated to change from AES-128 to some other cipher, another bit might be allocated to add a protocol feature (such as, e.g., support for measuring over multicast), yet another might be allocated to change a key derivation function, etc. The progression of versions is not a linear order, but rather a partial order. An implementation can implement any subset of these features (of course, features can be made mandatory to implement, e.g., new more secure ciphers if they are needed). Should a cipher with a different key size (say, a 256-bit key) become needed, a new key derivation function for OWAMP-Test keys would also be needed. The semantics of change in the cipher SHOULD then in the future be tied to the semantics of change in the key derivation function (KDF). One KDF that might be considered for the purpose might be a pseudo-random function (PRF) with appropriately sized output, such as 256 bits (perhaps HMAC-SHA256, if it is then still considered a secure PRF), which could then be used to derive the OWAMP-Test session keys from the OWAMP-Control session key by using the OWAMP-Control session key as the HMAC key and the SID as HMAC message. Note that the replacement scheme outlined above is trivially susceptible to downgrade attacks: a malicious party in the middle can flip modes bits as the mode is negotiated so that the oldest and weakest mode supported by the two parties is used. If this is deemed problematic at the time of cryptographic primitive replacement, the scheme might be augmented with a measure to prevent such an attack (by perhaps exchanging the modes again once a secure communications channel is established, comparing the two sets of mode words, and dropping the connection should they not match).6.8. Long-term Manually Managed Keys
OWAMP-Control uses long-term keys with manual management. These keys are used to automatically negotiate session keys for each OWAMP- Control session running in authenticated or encrypted mode. The number of these keys managed by a server scales linearly with (and,
in fact, is equal to) the number of administratively different users (perhaps particular humans, roles, or robots representing sites) that need to connect to this server. Similarly, the number of different manual keys managed by each client is the number of different servers that the client needs to connect to. This use of manual long-term keys is compliant with [BCP107].6.9. (Not) Using Time as Salt
A natural idea is to use the current time as salt when deriving session keys. Unfortunately, this appears to be too limiting. Although OWAMP is often run on hosts with well-synchronized clocks, it is also possible to run it on hosts with clocks completely untrained. The delays obtained thus are, of course, not directly usable; however, some metrics, such as unidirectional loss, reordering, measures of congestion such as the median delay minus minimum, and many others are usable directly and immediately (and improve upon the information that would have been provided by a round-trip measurement). Further, even delay information can be useful with appropriate post-processing. Indeed, one can even argue that running the clocks free and post-processing the results of a mesh of measurements will result in better accuracy, as more information is available a posteriori and correlation of data from different hosts is possible in post-processing, but not with online clock training. Given this, time is not used as salt in key derivation.6.10. The Use of AES-CBC and HMAC
OWAMP relies on AES-CBC for confidentiality and on HMAC-SHA1 truncated to 128 bits for message authentication. Random IV choice is important for prevention of a codebook attack on the first block (it should also be noted that, with its 128-bit block size, AES is more resistant to codebook attacks than are ciphers with shorter blocks; we use random IV anyway). HMAC MUST verify. It is crucial to check for this before using the message; otherwise, existential forgery becomes possible. The complete message for which HMAC verification fails MUST be discarded (both for short messages consisting of a few blocks and potentially for long messages, such as a response to the Fetch-Session command). If such a message is part of OWAMP-Control, the connection MUST be dropped. Since OWAMP messages can have different numbers of blocks, the existential forgery attack described in example 9.62 of [MENEZES]
becomes a concern. To prevent it (and to simplify implementation), the length of any message becomes known after decrypting its first block. A special case is the first (fixed-length) message sent by the client. There, the token is a concatenation of the 128-bit challenge (transmitted by the server in the clear), a 128-bit AES Session-key (generated randomly by the client, encrypted with AES-CBC with IV=0), and a 256-bit HMAC-SHA1 Session-key used for authentication. Since IV=0, the challenge (a single cipher block) is simply encrypted with the secret key. Therefore, we rely on resistance of AES to chosen plaintext attacks (as the challenge could be substituted by an attacker). It should be noted that the number of blocks of chosen plaintext an attacker can have encrypted with the secret key is limited by the number of sessions the client wants to initiate. An attacker who knows the encryption of a server's challenge can produce an existential forgery of the session key and thus disrupt the session; however, any attacker can disrupt a session by corrupting the protocol messages in an arbitrary fashion. Therefore, no new threat is created here; nevertheless, we require that the server never issues the same challenge twice. (If challenges are generated randomly, a repetition would occur, on average, after 2^64 sessions; we deem this satisfactory as this is enough even for an implausibly busy server that participates in 1,000,000 sessions per second to go without repetitions for more than 500 centuries.) With respect to the second part of the token, an attacker can produce an existential forgery of the session key by modifying the second half of the client's token while leaving the first part intact. This forgery, however, would be immediately discovered by the client when the HMAC on the server's next message (acceptance or rejection of the connection) does not verify.7. Acknowledgements
We would like to thank Guy Almes, Mark Allman, Jari Arkko, Hamid Asgari, Steven Van den Berghe, Eric Boyd, Robert Cole, Joan Cucchiara, Stephen Donnelly, Susan Evett, Sam Hartman, Kaynam Hedayat, Petri Helenius, Scott Hollenbeck, Russ Housley, Kitamura Yasuichi, Daniel H. T. R. Lawson, Will E. Leland, Bruce A. Mah, Allison Mankin, Al Morton, Attila Pasztor, Randy Presuhn, Matthew Roughan, Andy Scherrer, Henk Uijterwaal, and Sam Weiler for their comments, suggestions, reviews, helpful discussion and proof-reading.8. IANA Considerations
IANA has allocated a well-known TCP port number (861) for the OWAMP- Control part of the OWAMP protocol.
9. Internationalization Considerations
The protocol does not carry any information in a natural language, with the possible exception of the KeyID in OWAMP-Control, which is encoded in UTF-8.10. References
10.1. Normative References
[AES] Advanced Encryption Standard (AES), http://csrc.nist.gov/encryption/aes/ [BCP107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic Key Management", BCP 107, RFC 4107, June 2005. [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. [RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis, "Framework for IP Performance Metrics", RFC 2330, May 1998. [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One- way Delay Metric for IPPM", RFC 2679, September 1999. [RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One- way Packet Loss Metric for IPPM", RFC 2680, September 1999. [RFC2836] Brim, S., Carpenter, B., and F. Le Faucheur, "Per Hop Behavior Identification Codes", RFC 2836, May 2000. [RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography Specification Version 2.0", RFC 2898, September 2000.
10.2. Informative References
[APAN] Z. Shu and K. Kobayashi, "HOTS: An OWAMP-Compliant Hardware Packet Timestamper", In Proceedings of PAM 2005, http://www.springerlink.com/index/ W4GBD39YWC11GQTN.pdf [BRIX] Brix Networks, http://www.brixnet.com/ [ZIGG] J. H. Ahrens, U. Dieter, "Computer methods for sampling from the exponential and normal distributions", Communications of ACM, volume 15, issue 10, 873-882, 1972. http://doi.acm.org/10.1145/355604.361593 [MENEZES] A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone, Handbook of Applied Cryptography, CRC Press, revised reprint with updates, 1997. [KNUTH] D. Knuth, The Art of Computer Programming, vol.2, 3rd edition, 1998. [Abilene] One-way Latency Measurement (OWAMP), http://e2epi.internet2.edu/owamp/ [RIJN] Reference ANSI C Implementation of Rijndael, http://www.esat.kuleuven.ac.be/~rijmen/ rijndael/rijndaelref.zip [RIPE] RIPE NCC Test-Traffic Measurements home, http://www.ripe.net/test-traffic/. [SURVEYOR] Surveyor Home Page, http://www.advanced.org/surveyor/. [SURVEYOR-INET] S. Kalidindi and M. Zekauskas, "Surveyor: An Infrastructure for Network Performance Measurements", Proceedings of INET'99, June 1999. http://www.isoc.org/inet99/proceedings/4h/4h_2.htm [RFC1305] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation and Analysis", RFC 1305, March 1992. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 3546, June 2003. [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.
Appendix A: Sample C Code for Exponential Deviates
The values in array Q[] are the exact values that MUST be used by all implementations (see Sections 5.1 and 5.2). This appendix only serves for illustrative purposes. /* ** Example usage: generate a stream of exponential (mean 1) ** random quantities (ignoring error checking during initialization). ** If a variate with some mean mu other than 1 is desired, the output ** of this algorithm can be multiplied by mu according to the rules ** of arithmetic we described. ** Assume that a 16-octet 'seed' has been initialized ** (as the shared secret in OWAMP, for example) ** unsigned char seed[16]; ** OWPrand_context next; ** (initialize state) ** OWPrand_context_init(&next, seed); ** (generate a sequence of exponential variates) ** while (1) { ** u_int64_t num = OWPexp_rand64(&next); <do something with num here> ... ** } */ #include <stdlib.h> typedef u_int64_t u_int64_t; /* (K - 1) is the first k such that Q[k] > 1 - 1/(2^32). */ #define K 12 #define BIT31 0x80000000UL /* See if first bit in the lower 32 bits is zero. */ #define MASK32(n) ((n) & 0xFFFFFFFFUL) #define EXP2POW32 0x100000000ULL typedef struct OWPrand_context { unsigned char counter[16];/* Counter (network byte order).*/ keyInstance key; /* Key to encrypt the counter.*/ unsigned char out[16]; /* The encrypted block.*/
} OWPrand_context;
/*
** The array has been computed according to the formula:
**
** Q[k] = (ln2)/(1!) + (ln2)^2/(2!) + ... + (ln2)^k/(k!)
**
** as described in algorithm S. (The values below have been
** multiplied by 2^32 and rounded to the nearest integer.)
** These exact values MUST be used so that different implementation
** produce the same sequences.
*/
static u_int64_t Q[K] = {
0, /* Placeholder - so array indices start from 1. */
0xB17217F8,
0xEEF193F7,
0xFD271862,
0xFF9D6DD0,
0xFFF4CFD0,
0xFFFEE819,
0xFFFFE7FF,
0xFFFFFE2B,
0xFFFFFFE0,
0xFFFFFFFE,
0xFFFFFFFF
};
/* this element represents ln2 */
#define LN2 Q[1]
/*
** Convert an unsigned 32-bit integer into a u_int64_t number.
*/
u_int64_t
OWPulong2num64(u_int32_t a)
{
return ((u_int64_t)1 << 32) * a;
}
/*
** Arithmetic functions on u_int64_t numbers.
*/
/*
** Addition.
*/
u_int64_t
OWPnum64_add(u_int64_t x, u_int64_t y)
{
return x + y;
}
/*
** Multiplication. Allows overflow. Straightforward implementation
** of Algorithm 4.3.1.M (p.268) from [KNUTH].
*/
u_int64_t
OWPnum64_mul(u_int64_t x, u_int64_t y)
{
unsigned long w[4];
u_int64_t xdec[2];
u_int64_t ydec[2];
int i, j;
u_int64_t k, t, ret;
xdec[0] = MASK32(x);
xdec[1] = MASK32(x>>32);
ydec[0] = MASK32(y);
ydec[1] = MASK32(y>>32);
for (j = 0; j < 4; j++)
w[j] = 0;
for (j = 0; j < 2; j++) {
k = 0;
for (i = 0; ; ) {
t = k + (xdec[i]*ydec[j]) + w[i + j];
w[i + j] = t%EXP2POW32;
k = t/EXP2POW32;
if (++i < 2)
continue;
else {
w[j + 2] = k;
break;
}
}
}
ret = w[2];
ret <<= 32;
return w[1] + ret;
}
/*
** Seed the random number generator using a 16-byte quantity 'seed'
** (== the session ID in OWAMP). This function implements step U1
** of algorithm Unif.
*/
void
OWPrand_context_init(OWPrand_context *next, unsigned char *seed)
{
int i;
/* Initialize the key */
rijndaelKeyInit(next->key, seed);
/* Initialize the counter with zeros */
memset(next->out, 0, 16);
for (i = 0; i < 16; i++)
next->counter[i] = 0UL;
}
/*
** Random number generating functions.
*/
/*
** Generate and return a 32-bit uniform random value (saved in the
**less significant half of the u_int64_t). This function implements
**steps U2-U4 of the algorithm Unif.
*/
u_int64_t
OWPunif_rand64(OWPrand_context *next)
{
int j;
u_int8_t *buf;
u_int64_t ret = 0;
/* step U2 */
u_int8_t i = next->counter[15] & (u_int8_t)3;
if (!i)
rijndaelEncrypt(next->key, next->counter, next->out);
/* Step U3. Increment next.counter as a 16-octet single
quantity in network byte order for AES counter mode. */
for (j = 15; j >= 0; j--)
if (++next->counter[j])
break;
/* Step U4. Do output. The last 4 bytes of ret now contain
the random integer in network byte order */
buf = &next->out[4*i];
for (j=0; j<4; j++) {
ret <<= 8;
ret += *buf++;
}
return ret;
}
/*
** Generate an exponential deviate with mean 1.
*/
u_int64_t
OWPexp_rand64(OWPrand_context *next)
{
unsigned long i, k;
u_int32_t j = 0;
u_int64_t U, V, J, tmp;
/* Step S1. Get U and shift */
U = OWPunif_rand64(next);
while ((U & BIT31) && (j < 32)) { /* Shift until first 0. */
U <<= 1;
j++;
}
/* Remove the 0 itself. */
U <<= 1;
U = MASK32(U); /* Keep only the fractional part. */
J = OWPulong2num64(j);
/* Step S2. Immediate acceptance? */
if (U < LN2) /* return (j*ln2 + U) */
return OWPnum64_add(OWPnum64_mul(J, LN2), U);
/* Step S3. Minimize. */
for (k = 2; k < K; k++)
if (U < Q[k])
break;
V = OWPunif_rand64(next);
for (i = 2; i <= k; i++) {
tmp = OWPunif_rand64(next);
if (tmp < V)
V = tmp;
}
/* Step S4. Return (j+V)*ln2 */
return OWPnum64_mul(OWPnum64_add(J, V), LN2);
}
Appendix B: Test Vectors for Exponential Deviates
It is important that the test schedules generated by different
implementations from identical inputs be identical. The non-trivial
part is the generation of pseudo-random exponentially distributed
deviates. To aid implementors in verifying interoperability, several
test vectors are provided. For each of the four given 128-bit values
of SID represented as hexadecimal numbers, 1,000,000 exponentially
distributed 64-bit deviates are generated as described above. As
they are generated, they are all added to each other. The sum of all
1,000,000 deviates is given as a hexadecimal number for each SID. An
implementation MUST produce exactly these hexadecimal numbers. To
aid in the verification of the conversion of these numbers to values
of delay in seconds, approximate values are given (assuming
lambda=1). An implementation SHOULD produce delay values in seconds
that are close to the ones given below.
SID = 0x2872979303ab47eeac028dab3829dab2
SUM[1000000] = 0x000f4479bd317381 (1000569.739036 seconds)
SID = 0x0102030405060708090a0b0c0d0e0f00
SUM[1000000] = 0x000f433686466a62 (1000246.524512 seconds)
SID = 0xdeadbeefdeadbeefdeadbeefdeadbeef
SUM[1000000] = 0x000f416c8884d2d3 (999788.533277 seconds)
SID = 0xfeed0feed1feed2feed3feed4feed5ab
SUM[1000000] = 0x000f3f0b4b416ec8 (999179.293967 seconds)
Authors' Addresses
Stanislav Shalunov Internet2 1000 Oakbrook Drive, Suite 300 Ann Arbor, MI 48104 EMail: shalunov@internet2.edu WWW: http://www.internet2.edu/~shalunov/ Benjamin Teitelbaum Internet2 1000 Oakbrook Drive, Suite 300 Ann Arbor, MI 48104 EMail: ben@internet2.edu WWW: http://people.internet2.edu/~ben/ Anatoly Karp Computer Sciences Department University of Wisconsin-Madison Madison, WI 53706 EMail: akarp@cs.wisc.edu Jeff W. Boote Internet2 1000 Oakbrook Drive, Suite 300 Ann Arbor, MI 48104 EMail: boote@internet2.edu Matthew J. Zekauskas Internet2 1000 Oakbrook Drive, Suite 300 Ann Arbor, MI 48104 EMail: matt@internet2.edu
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