Internet Engineering Task Force David Wetherall
INTERNET DRAFT David Ely
draft-ietf-tsvwg-tcp-nonce-00.txt Neil Spring
University of Washington
January, 2001
Expires: July, 2001
Robust ECN Signaling with Nonces
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This note describes a simple modification to ECN that protects
against accidental or malicious concealment of marked packets from
the TCP sender. This is valuable because it improves the robustness
of congestion control by preventing receivers from exploiting ECN to
gaining an unfair share of the bandwidth. The mechanism uses a
slightly different encoding than the existing two ECN bits in the IP
header, and also requires one additional bit in the TCP header. It is
computationally efficient for both routers and hosts.
1. Introduction
The correct operation of ECN requires the cooperation of the receiver
to return Congestion Experienced signals to the sender, but the
protocol lacks a mechanism to enforce this cooperation. This raises
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the possibility that an unscrupulous or poorly implemented receiver
could always clear ECN-Echo and simply not return congestion signals
to the sender. This in turn would give the receivers a performance
advantage at the expense of competing connections that behave
properly. More generally, any device along the path (NAT box,
firewall, QOS bandwidth shapers, and so forth) could remove
congestion marks with impunity.
The above behaviors may or may not constitute a threat to the
operation of congestion control in the Internet. However, given of
the central role of congestion control, we feel it is prudent to
design the ECN signaling loop to be robust against as many threats as
possible. In this way ECN can provide a clear incentive for
improvement over the prior state-of-the-art without potential
incentives for abuse. In this note, we show how this can be achieved
while at the same time keeping the protocol simple and efficient.
Our signaling mechanism uses random one bit quantities to allow the
sender to verify that the receiver has implemented ECN signaling
correctly and that there is no other interference that conceals
marked (or dropped) packets in the signaling path. This provides
protection against both implementation errors and deliberate abuse.
In developing the nonce signaling mechanism we met the following
goals:
- It provides an additional check but does not change other aspects
of ECN, and nor does it reduce the benefits of ECN for behaving
receivers.
- It catches a misbehaving receiver with a high probability, and
never convicts an innocent receiver
- It is cheap in terms of per-packet overhead (one TCP header bit
more than the existing ECN mechanism) and processing
requirements.
- It is simple and, to the best of our knowledge, not prone to
other attacks
The rest of this note describes the mechanism, first with an overview
and then in terms of more detailed behavior at senders, routers and
receivers.
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2. Overview of Solution
Our scheme builds on the existing ECN-Echo and CWR signaling
mechanism. Familiarity with ECN is assumed in this note. Also, for
simplicity, we describe our scheme in one direction only, though it
is run in both directions in parallel.
In a nutshell, our approach is to detect misbehavior by attaching a
random one bit nonce value to packets at the sender and having
acknowledgments from the receiver echo the nonce information that is
received. At routers, packet marking becomes the process of erasing
the nonce value. Therefore, once a packet has been marked by a
router, it cannot be unmarked by any party without successfully
guessing the value of the erased nonce. Thus receipt of correct nonce
information from the receiver provides the sender with a
probabilistic proof-of- receipt check for unmarked packets. The check
is used by the TCP sender to verify that the ECN-Echo bit is being
set correctly and that congestion indications in the form of marked
(or dropped) packets are not being concealed. Because one bit of
information is returned with each acknowledgement, senders have a
50-50 chance of catching a lying receiver every time they perform a
check. Because the check for each acknowledgement is an independent
trial it is highly likely that cheaters will be caught quickly if
there are repeated packet marks.
There are several areas of detail missing from the preceding high-
level description. We mention those areas to complete the overview.
First, the nonce values are echoed in the form of nonce sums. Each
nonce sum is carried in an acknowledgement, and represents the one
bit sum (XOR or parity) of nonces over the byte range represented by
the acknowledgement. To understand why the sum is used, rather than
individual echoes, consider the following argument. If every packet
were reliably ACKed, then the nonce carried in the unmarked packet
could simply be echoed. This would probabilistically prove to the
sender that the receiver received the packet and the packet was
unmarked. However, ACKs are not carried across the network reliably,
and not every packet is ACKed. In this case, the sender cannot
distinguish a lost ACK from one that was never sent in order to
conceal a marked packet. It would require additional mechanism,
beyond that used in TCP, to convey the nonce bits reliably. Instead,
we send the nonce sum that corresponds to the cumulative ACK. This
sum prevents individual marked packets from being concealed by not
acknowledging them. Note that because they are both one bit
quantities, the sum is no easier to guess than the individual nonces.
Second, resynchronization of the sender and receiver sums is needed
after congestion has occurred and packets have been marked. When
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packets are marked, the nonce is cleared, and the sum of the nonces
at the receiver will no longer reflect the sum at the sender. While
this is conceptually fixed by having the receiver send a series of
partial sums for the ranges of unmarked packets that it has received,
this solution is clumsy because the required range information is not
already being sent. Fortunately, there is a simple solution that
does not require range information because ECN congestion indications
do not need to indicate the particular packets that were marked. We
observe that once nonces have been lost, the difference between
sender and receiver nonce sums will be fixed until there is further
loss. This means that it is possible to resynchronize the sender and
receiver after congestion by having the sender set its nonce sum to
that of the receiver. Because congestion indications do not need to
be conveyed more frequently than once per round trip, we suspend
checking while the CWR signal is being delivered and acknowledged by
the receiver. We reset the sender nonce sum to the receiver sum when
new data is acknowledged. This scheme is simple and has the benefit
that the receiver is not explicitly involved in the re-
synchronization process.
Third, we need to reconcile the nonces that are sent with packet with
acknowledgements that cover byte ranges. Acknowledged byte boundaries
need not match the transmitted boundaries, and during retransmissions
information can be resent with different byte boundaries. To handle
these factors, we compute nonces and nonce sums using an underlying
mapping of byte ranges to nonce values. Both sender and receiver
understand this mapping, and can convert to and from the nonces
carried on individual packets.
The next sections describe the detailed behavior of senders, routers
and receivers. We start with sender transmit behavior, and work our
way around the ECN signaling loop until we arrive back at sender
receive behavior. Comments in parenthesis highlight the changes
between the nonce mechanism and the existing ECN specification.
3. Sender Behavior (Transmit)
Senders in our scheme manage CWR and ECN-Echo as before. In addition
they must place nonces on packets as they are transmitted and check
the validity of the nonce sums on packets as they are received. This
section describes the transmit process.
To place a one bit nonce value on every IP packet requires first of
all a way to encode these bits in IP packets. We use the following
encoding of the ECN bits to identify different packet states. This
encoding must be understood by all ECN capable senders, routers, and
receivers in our scheme. (The second state below is currently unused
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in the existing ECN specification, while the other states retain
their existing meanings.)
00 = ECN incapable
10 = ECN capable, unmarked (nonce = 0)
01 = ECN capable, unmarked (nonce = 1)
11 = ECN capable, marked (nonce lost)
Next, we require a simple way to map nonces to transmitted TCP
packets in a manner that is compatible with checking the nonce sum on
received TCP acknowledgements. This is complicated by several
factors. Nonces are sent per packet but acknowledgements cover byte
ranges that do not necessarily correspond to the original packet
ranges; this can depend on implementation buffering strategies. In
the case of retransmissions, the boundary of retransmitted packets
need not correspond to the original transmissions either (because of
path MTU changes, retransmission batching, and so forth). Finally,
there is ambiguity at the sender as to whether the original or
retransmitted packet was received. It is important that our
implementation behave correctly even in these rare cases so that a
receiver is never incorrectly labeled as misbehaving.
We associate nonce values with byte ranges instead of individual
packets to avoid these difficulties. Starting from the initial
sequence number, each block of SMSS bytes (the maximum segment size)
in the TCP byte stream is associated with a single pseudorandom nonce
bit. The byte range of the packet determines what nonce value it
will carry. If the packet, either original or a retransmission, spans
multiple blocks, we use the block in which the final byte of the
packet resides to determine which nonce value to transmit with the
packet. A series of small packets will carry the same nonce value
until an entire block's worth of SMSS bytes has been transmitted.
This is a useful tradeoff because sending partial packets makes a
flow less likely to cause congestion. Since no packet can carry more
than SMSS bytes, each block's nonce bit will be carried in at least
one packet.
The following table provides an example of how we decompose a byte
stream into blocks and how we assign nonce values to each block
(assuming a block size of 1460 bytes):
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-------------------------------------------------------
| Bytes: | 1...1460 | 1461...2920 | 2921 ... 4380 |
-------------------------------------------------------
| Nonce: | 1 | 1 | 0 |
-------------------------------------------------------
| Nonce Sum: | 1 | 0 | 1 |
-------------------------------------------------------
4. Router Behavior
Routers must be able to identify packets sent by ECN capable
transports and mark them if indicated by active queue management. To
mark packets, routers change either of the unmarked states to the
single marked state. (The operation of marking has changed only in
that routers now need to recognize two states as meaning not marked
and so is still straightforward.) This erases the state of the
original nonce carried with the packet, which is key to our scheme.
Neither the receiver nor any other party can now unmark the packet
without successfully guessing the value of the original nonce.
5. Receiver Behavior (Receive and Transmit)
In addition to distinguishing marked and unmarked packets and setting
the ECN- Echo flag as before, receivers in our scheme maintain a
nonce sum as packets arrive, and return the sum that corresponds to a
particular acknowledgment with the acknowledgment.
To maintain the nonce sum, receivers use the same mapping as the
sender to convert the nonces carried in unmarked packets to the
nonces of the underlying blocks. These nonce values are summed over
the byte range covered by the acknowledgement. Computing this sum
correctly when packets of size SMSS are sent requires that all
packets up to the one acknowledged be received. New sums are computed
by taking the old value and XORing it with a new nonce. That is, the
sum is also a one bit quantity, and old nonce state does not need to
be maintained.
In the case of marked packets, one or more nonce values may be
unknown to the receiver. In this case the missing nonce values are
ignored when calculating the sum (or equivalently a value of zero is
assumed) and ECN-Echo will be set to signal congestion to the sender.
Returning the nonce sum corresponding to a given acknowledgement is
straightforward. It is carried in a single bit in the TCP header.
(This bit is in addition the CWR and ECN-Echo bits and would require
one of the reserved bits to be allocated.)
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These nonce sums are checked for validity at the sender, as described
below.
6. Sender Behavior (Receive)
This section completes the description of sender behavior by
describing how senders check the validity of the nonce sums.
Checking is straightforward and is performed every time an
acknowledgement is received, except during congestion recovery. Given
the byte range covered by an acknowledgement and the mapping between
bytes and nonces, the sender is able to compute the correct nonce
sum. Minimal sender state is needed to do this because old nonce
values can be discarded as acknowledgments and the sum advance.
Checking consists of simply comparing the correct nonce sum and that
carried in the acknowledgement.
If ECN-Echo is not set, the receiver claims to have received no
marked packets, and can therefore compute the correct nonce sum. To
cheat, the receiver must successfully guess the sum of the nonces
that it did not receive (because at least one packet was marked and
the corresponding nonce was erased). Provided the individual nonces
are equally likely to be 0 or 1, their sum is equally likely to be 0
or 1. In other words, any guess is equally likely to be wrong and
has a 50-50 chance of being caught by the sender. Because each
acknowledgement (that covers a new block) is an independent trial, a
cheating receiver is highly likely to be caught after a small number
of lies.
If ECN-Echo is set, the receiver is sending a congestion signal and
it is not necessary to check the nonce sum. The congestion window
will be halved, CWR will be set on the next packet with new data
sent, and ECN-Echo will be cleared once the CWR signal is received.
During this recovery process, the sum may be incorrect because one or
more nonces were not received. This does not matter during recovery,
because TCP invokes congestion mechanisms at most once per RTT,
whether there are one or more losses during that period. However,
after recovery, it is necessary to re-synchronize the sender and
receiver nonce sums so that further acknowledgments can be checked.
If might be possible to send the missing nonces to the receiver, but
this would be cumbersome because TCP lacks the mechanism to do so
conveniently. Instead, we observe that if there are no more marked
packets, the sender and receiver sums should differ by a constant
amount. This leads to a simple re-synchronization mechanism where the
sender resets its nonce sum to that of the receiver when it receives
an acknowledgment for new data sent after the congestion window was
reduced. In most instances, this will be the first acknowledgement
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without the ECN-Echo flag set.
A separate issue is the penalty for misbehavior that is caught by
checking. During normal operation, both with and without packet
marks and drops, no misbehavior will be uncovered unless some party
after the marking router is behaving incorrectly. A simple remedy in
this case would be to disable ECN at the sender, that is, not mark
packets as ECN capable. This simultaneously deprives the receiver of
the benefits of ECN and relieves the sender of the need to monitor
the receiver. However, an additional consideration is that the nonce
checking mechanism provides robustness beyond checking that marked
packets are signaled to the sender. It also ensures that dropped
packets cannot be concealed from the sender (because their nonces
have been lost). Drops could potentially be concealed by a faulty TCP
implementation, certain attacks, or even a hypothetical a TCP
accelerator willing to gamble that it can either successfully ``fast
start'' to a preset bandwidth quickly, retry with another connection,
or provide reliability at the application level. If robustness
against these faults is considered valuable (as opposed to simply
detecting a faulty ECN implementation) then it is not clear that the
nonce mechanism should be turned off. Instead, a penalty such as
reducing the congestion window by a factor of 4 may be preferable.
This would provide continued checking while punishing faulty
operation. Luckily, this issue is separate from the checking
mechanism and does not need to be handled uniformly by senders.
7. Conclusion
We have described a simple modification to the ECN signaling
mechanism that improves its robustness by preventing receivers from
concealing marked (or dropped) packets. The intent of this work is to
help improve the robustness of congestion control in the Internet.
The modification is retains the character and simplicity of existing
ECN signaling. It is also practical for deployment in the Internet.
It requires two bits in the IP header (ECT and CE with a slightly
different encoding) and one additional bit in the TCP header (as well
as CWR and ECN-Echo) and has simple processing rules.
Acknowledgements
This note grew out of research done by Stefan Savage, David Ely,
David Wetherall, Tom Anderson and Neil Spring. We are grateful for
feedback from Sally Floyd.
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Authors' Addresses
David Wetherall
Email: djw@cs.washington.edu
Phone +1 (206) 616 4367
David Ely
Email: ely@cs.washington.edu
Neil Spring
Email: nspring@cs.washington.edu
Computer Science and Engineering, 352350
University of Washington
Seattle, WA 98195-2350
Send comments by electronic mail to all three authors.
This draft was created in January 2001.
It expires July 2001.
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