6. Support from the Transport Protocol
ECN requires support from the transport protocol, in addition to the
functionality given by the ECN field in the IP packet header. The
transport protocol might require negotiation between the endpoints
during setup to determine that all of the endpoints are ECN-capable,
so that the sender can set the ECT codepoint in transmitted packets.
Second, the transport protocol must be capable of reacting
appropriately to the receipt of CE packets. This reaction could be
in the form of the data receiver informing the data sender of the
received CE packet (e.g., TCP), of the data receiver unsubscribing to
a layered multicast group (e.g., RLM [MJV96]), or of some other
action that ultimately reduces the arrival rate of that flow on that
congested link. CE packets indicate persistent rather than transient
congestion (see Section 5.1), and hence reactions to the receipt of
CE packets should be those appropriate for persistent congestion.
This document only addresses the addition of ECN Capability to TCP,
leaving issues of ECN in other transport protocols to further
research. For TCP, ECN requires three new pieces of functionality:
negotiation between the endpoints during connection setup to
determine if they are both ECN-capable; an ECN-Echo (ECE) flag in the
TCP header so that the data receiver can inform the data sender when
a CE packet has been received; and a Congestion Window Reduced (CWR)
flag in the TCP header so that the data sender can inform the data
receiver that the congestion window has been reduced. The support
required from other transport protocols is likely to be different,
particularly for unreliable or reliable multicast transport
protocols, and will have to be determined as other transport
protocols are brought to the IETF for standardization.
In a mild abuse of terminology, in this document we refer to `TCP
packets' instead of `TCP segments'.
The following sections describe in detail the proposed use of ECN in
TCP. This proposal is described in essentially the same form in
[Floyd94]. We assume that the source TCP uses the standard congestion
control algorithms of Slow-start, Fast Retransmit and Fast Recovery
This proposal specifies two new flags in the Reserved field of the
TCP header. The TCP mechanism for negotiating ECN-Capability uses
the ECN-Echo (ECE) flag in the TCP header. Bit 9 in the Reserved
field of the TCP header is designated as the ECN-Echo flag. The
location of the 6-bit Reserved field in the TCP header is shown in
Figure 4 of RFC 793 [RFC793] (and is reproduced below for
completeness). This specification of the ECN Field leaves the
Reserved field as a 4-bit field using bits 4-7.
To enable the TCP receiver to determine when to stop setting the
ECN-Echo flag, we introduce a second new flag in the TCP header, the
CWR flag. The CWR flag is assigned to Bit 8 in the Reserved field of
the TCP header.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
| | | U | A | P | R | S | F |
| Header Length | Reserved | R | C | S | S | Y | I |
| | | G | K | H | T | N | N |
Figure 3: The old definition of bytes 13 and 14 of the TCP
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
| | | C | E | U | A | P | R | S | F |
| Header Length | Reserved | W | C | R | C | S | S | Y | I |
| | | R | E | G | K | H | T | N | N |
Figure 4: The new definition of bytes 13 and 14 of the TCP
Thus, ECN uses the ECT and CE flags in the IP header (as shown in
Figure 1) for signaling between routers and connection endpoints, and
uses the ECN-Echo and CWR flags in the TCP header (as shown in Figure
4) for TCP-endpoint to TCP-endpoint signaling. For a TCP connection,
a typical sequence of events in an ECN-based reaction to congestion
is as follows:
* An ECT codepoint is set in packets transmitted by the sender to
indicate that ECN is supported by the transport entities for
* An ECN-capable router detects impending congestion and detects
that an ECT codepoint is set in the packet it is about to drop.
Instead of dropping the packet, the router chooses to set the CE
codepoint in the IP header and forwards the packet.
* The receiver receives the packet with the CE codepoint set, and
sets the ECN-Echo flag in its next TCP ACK sent to the sender.
* The sender receives the TCP ACK with ECN-Echo set, and reacts to
the congestion as if a packet had been dropped.
* The sender sets the CWR flag in the TCP header of the next
packet sent to the receiver to acknowledge its receipt of and
reaction to the ECN-Echo flag.
The negotiation for using ECN by the TCP transport entities and the
use of the ECN-Echo and CWR flags is described in more detail in the
6.1.1 TCP Initialization
In the TCP connection setup phase, the source and destination TCPs
exchange information about their willingness to use ECN. Subsequent
to the completion of this negotiation, the TCP sender sets an ECT
codepoint in the IP header of data packets to indicate to the network
that the transport is capable and willing to participate in ECN for
this packet. This indicates to the routers that they may mark this
packet with the CE codepoint, if they would like to use that as a
method of congestion notification. If the TCP connection does not
wish to use ECN notification for a particular packet, the sending TCP
sets the ECN codepoint to not-ECT, and the TCP receiver ignores the
CE codepoint in the received packet.
For this discussion, we designate the initiating host as Host A and
the responding host as Host B. We call a SYN packet with the ECE and
CWR flags set an "ECN-setup SYN packet", and we call a SYN packet
with at least one of the ECE and CWR flags not set a "non-ECN-setup
SYN packet". Similarly, we call a SYN-ACK packet with only the ECE
flag set but the CWR flag not set an "ECN-setup SYN-ACK packet", and
we call a SYN-ACK packet with any other configuration of the ECE and
CWR flags a "non-ECN-setup SYN-ACK packet".
Before a TCP connection can use ECN, Host A sends an ECN-setup SYN
packet, and Host B sends an ECN-setup SYN-ACK packet. For a SYN
packet, the setting of both ECE and CWR in the ECN-setup SYN packet
is defined as an indication that the sending TCP is ECN-Capable,
rather than as an indication of congestion or of response to
congestion. More precisely, an ECN-setup SYN packet indicates that
the TCP implementation transmitting the SYN packet will participate
in ECN as both a sender and receiver. Specifically, as a receiver,
it will respond to incoming data packets that have the CE codepoint
set in the IP header by setting ECE in outgoing TCP Acknowledgement
(ACK) packets. As a sender, it will respond to incoming packets that
have ECE set by reducing the congestion window and setting CWR when
appropriate. An ECN-setup SYN packet does not commit the TCP sender
to setting the ECT codepoint in any or all of the packets it may
transmit. However, the commitment to respond appropriately to
incoming packets with the CE codepoint set remains even if the TCP
sender in a later transmission, within this TCP connection, sends a
SYN packet without ECE and CWR set.
When Host B sends an ECN-setup SYN-ACK packet, it sets the ECE flag
but not the CWR flag. An ECN-setup SYN-ACK packet is defined as an
indication that the TCP transmitting the SYN-ACK packet is ECN-
Capable. As with the SYN packet, an ECN-setup SYN-ACK packet does
not commit the TCP host to setting the ECT codepoint in transmitted
The following rules apply to the sending of ECN-setup packets within
a TCP connection, where a TCP connection is defined by the standard
rules for TCP connection establishment and termination.
* If a host has received an ECN-setup SYN packet, then it MAY send
an ECN-setup SYN-ACK packet. Otherwise, it MUST NOT send an
ECN-setup SYN-ACK packet.
* A host MUST NOT set ECT on data packets unless it has sent at
least one ECN-setup SYN or ECN-setup SYN-ACK packet, and has
received at least one ECN-setup SYN or ECN-setup SYN-ACK packet,
and has sent no non-ECN-setup SYN or non-ECN-setup SYN-ACK
packet. If a host has received at least one non-ECN-setup SYN
or non-ECN-setup SYN-ACK packet, then it SHOULD NOT set ECT on
* If a host ever sets the ECT codepoint on a data packet, then
that host MUST correctly set/clear the CWR TCP bit on all
subsequent packets in the connection.
* If a host has sent at least one ECN-setup SYN or ECN-setup SYN-
ACK packet, and has received no non-ECN-setup SYN or non-ECN-
setup SYN-ACK packet, then if that host receives TCP data
packets with ECT and CE codepoints set in the IP header, then
that host MUST process these packets as specified for an ECN-
* A host that is not willing to use ECN on a TCP connection SHOULD
clear both the ECE and CWR flags in all non-ECN-setup SYN and/or
SYN-ACK packets that it sends to indicate this unwillingness.
Receivers MUST correctly handle all forms of the non-ECN-setup
SYN and SYN-ACK packets.
* A host MUST NOT set ECT on SYN or SYN-ACK packets.
A TCP client enters TIME-WAIT state after receiving a FIN-ACK, and
transitions to CLOSED state after a timeout. Many TCP
implementations create a new TCP connection if they receive an in-
window SYN packet during TIME-WAIT state. When a TCP host enters
TIME-WAIT or CLOSED state, it should ignore any previous state about
the negotiation of ECN for that connection.
220.127.116.11. Middlebox Issues
ECN introduces the use of the ECN-Echo and CWR flags in the TCP
header (as shown in Figure 3) for initialization. There exist some
faulty firewalls, load balancers, and intrusion detection systems in
the Internet that either drop an ECN-setup SYN packet or respond with
a RST, in the belief that such a packet (with these bits set) is a
signature for a port-scanning tool that could be used in a denial-
of-service attack. Some of the offending equipment has been
identified, and a web page [FIXES] contains a list of non-compliant
products and the fixes posted by the vendors, where these are
available. The TBIT web page [TBIT] lists some of the web servers
affected by this faulty equipment. We mention this in this document
as a warning to the community of this problem.
To provide robust connectivity even in the presence of such faulty
equipment, a host that receives a RST in response to the transmission
of an ECN-setup SYN packet MAY resend a SYN with CWR and ECE cleared.
This could result in a TCP connection being established without using
A host that receives no reply to an ECN-setup SYN within the normal
SYN retransmission timeout interval MAY resend the SYN and any
subsequent SYN retransmissions with CWR and ECE cleared. To overcome
normal packet loss that results in the original SYN being lost, the
originating host may retransmit one or more ECN-setup SYN packets
before giving up and retransmitting the SYN with the CWR and ECE bits
We note that in this case, the following example scenario is
(1) Host A: Sends an ECN-setup SYN.
(2) Host B: Sends an ECN-setup SYN/ACK, packet is dropped or delayed.
(3) Host A: Sends a non-ECN-setup SYN.
(4) Host B: Sends a non-ECN-setup SYN/ACK.
We note that in this case, following the procedures above, neither
Host A nor Host B may set the ECT bit on data packets. Further, an
important consequence of the rules for ECN setup and usage in Section
6.1.1 is that a host is forbidden from using the reception of ECT
data packets as an implicit signal that the other host is ECN-
18.104.22.168. Robust TCP Initialization with an Echoed Reserved Field
There is the question of why we chose to have the TCP sending the SYN
set two ECN-related flags in the Reserved field of the TCP header for
the SYN packet, while the responding TCP sending the SYN-ACK sets
only one ECN-related flag in the SYN-ACK packet. This asymmetry is
necessary for the robust negotiation of ECN-capability with some
deployed TCP implementations. There exists at least one faulty TCP
implementation in which TCP receivers set the Reserved field of the
TCP header in ACK packets (and hence the SYN-ACK) simply to reflect
the Reserved field of the TCP header in the received data packet.
Because the TCP SYN packet sets the ECN-Echo and CWR flags to
indicate ECN-capability, while the SYN-ACK packet sets only the ECN-
Echo flag, the sending TCP correctly interprets a receiver's
reflection of its own flags in the Reserved field as an indication
that the receiver is not ECN-capable. The sending TCP is not mislead
by a faulty TCP implementation sending a SYN-ACK packet that simply
reflects the Reserved field of the incoming SYN packet.
6.1.2. The TCP Sender
For a TCP connection using ECN, new data packets are transmitted with
an ECT codepoint set in the IP header. When only one ECT codepoint
is needed by a sender for all packets sent on a TCP connection,
ECT(0) SHOULD be used. If the sender receives an ECN-Echo (ECE) ACK
packet (that is, an ACK packet with the ECN-Echo flag set in the TCP
header), then the sender knows that congestion was encountered in the
network on the path from the sender to the receiver. The indication
of congestion should be treated just as a congestion loss in non-
ECN-Capable TCP. That is, the TCP source halves the congestion window
"cwnd" and reduces the slow start threshold "ssthresh". The sending
TCP SHOULD NOT increase the congestion window in response to the
receipt of an ECN-Echo ACK packet.
TCP should not react to congestion indications more than once every
window of data (or more loosely, more than once every round-trip
time). That is, the TCP sender's congestion window should be reduced
only once in response to a series of dropped and/or CE packets from a
single window of data. In addition, the TCP source should not
decrease the slow-start threshold, ssthresh, if it has been decreased
within the last round trip time. However, if any retransmitted
packets are dropped, then this is interpreted by the source TCP as a
new instance of congestion.
After the source TCP reduces its congestion window in response to a
CE packet, incoming acknowledgments that continue to arrive can
"clock out" outgoing packets as allowed by the reduced congestion
window. If the congestion window consists of only one MSS (maximum
segment size), and the sending TCP receives an ECN-Echo ACK packet,
then the sending TCP should in principle still reduce its congestion
window in half. However, the value of the congestion window is
bounded below by a value of one MSS. If the sending TCP were to
continue to send, using a congestion window of 1 MSS, this results in
the transmission of one packet per round-trip time. It is necessary
to still reduce the sending rate of the TCP sender even further, on
receipt of an ECN-Echo packet when the congestion window is one. We
use the retransmit timer as a means of reducing the rate further in
this circumstance. Therefore, the sending TCP MUST reset the
retransmit timer on receiving the ECN-Echo packet when the congestion
window is one. The sending TCP will then be able to send a new
packet only when the retransmit timer expires.
When an ECN-Capable TCP sender reduces its congestion window for any
reason (because of a retransmit timeout, a Fast Retransmit, or in
response to an ECN Notification), the TCP sender sets the CWR flag in
the TCP header of the first new data packet sent after the window
reduction. If that data packet is dropped in the network, then the
sending TCP will have to reduce the congestion window again and
retransmit the dropped packet.
We ensure that the "Congestion Window Reduced" information is
reliably delivered to the TCP receiver. This comes about from the
fact that if the new data packet carrying the CWR flag is dropped,
then the TCP sender will have to again reduce its congestion window,
and send another new data packet with the CWR flag set. Thus, the
CWR bit in the TCP header SHOULD NOT be set on retransmitted packets.
When the TCP data sender is ready to set the CWR bit after reducing
the congestion window, it SHOULD set the CWR bit only on the first
new data packet that it transmits.
[Floyd94] discusses TCP's response to ECN in more detail. [Floyd98]
discusses the validation test in the ns simulator, which illustrates
a wide range of ECN scenarios. These scenarios include the following:
an ECN followed by another ECN, a Fast Retransmit, or a Retransmit
Timeout; a Retransmit Timeout or a Fast Retransmit followed by an
ECN; and a congestion window of one packet followed by an ECN.
TCP follows existing algorithms for sending data packets in response
to incoming ACKs, multiple duplicate acknowledgments, or retransmit
timeouts [RFC2581]. TCP also follows the normal procedures for
increasing the congestion window when it receives ACK packets without
the ECN-Echo bit set [RFC2581].
6.1.3. The TCP Receiver
When TCP receives a CE data packet at the destination end-system, the
TCP data receiver sets the ECN-Echo flag in the TCP header of the
subsequent ACK packet. If there is any ACK withholding implemented,
as in current "delayed-ACK" TCP implementations where the TCP
receiver can send an ACK for two arriving data packets, then the
ECN-Echo flag in the ACK packet will be set to '1' if the CE
codepoint is set in any of the data packets being acknowledged. That
is, if any of the received data packets are CE packets, then the
returning ACK has the ECN-Echo flag set.
To provide robustness against the possibility of a dropped ACK packet
carrying an ECN-Echo flag, the TCP receiver sets the ECN-Echo flag in
a series of ACK packets sent subsequently. The TCP receiver uses the
CWR flag received from the TCP sender to determine when to stop
setting the ECN-Echo flag.
After a TCP receiver sends an ACK packet with the ECN-Echo bit set,
that TCP receiver continues to set the ECN-Echo flag in all the ACK
packets it sends (whether they acknowledge CE data packets or non-CE
data packets) until it receives a CWR packet (a packet with the CWR
flag set). After the receipt of the CWR packet, acknowledgments for
subsequent non-CE data packets do not have the ECN-Echo flag set. If
another CE packet is received by the data receiver, the receiver
would once again send ACK packets with the ECN-Echo flag set. While
the receipt of a CWR packet does not guarantee that the data sender
received the ECN-Echo message, this does suggest that the data sender
reduced its congestion window at some point *after* it sent the data
packet for which the CE codepoint was set.
We have already specified that a TCP sender is not required to reduce
its congestion window more than once per window of data. Some care
is required if the TCP sender is to avoid unnecessary reductions of
the congestion window when a window of data includes both dropped
packets and (marked) CE packets. This is illustrated in [Floyd98].
6.1.4. Congestion on the ACK-path
For the current generation of TCP congestion control algorithms, pure
acknowledgement packets (e.g., packets that do not contain any
accompanying data) MUST be sent with the not-ECT codepoint. Current
TCP receivers have no mechanisms for reducing traffic on the ACK-path
in response to congestion notification. Mechanisms for responding to
congestion on the ACK-path are areas for current and future research.
(One simple possibility would be for the sender to reduce its
congestion window when it receives a pure ACK packet with the CE
codepoint set). For current TCP implementations, a single dropped ACK
generally has only a very small effect on the TCP's sending rate.
6.1.5. Retransmitted TCP packets
This document specifies ECN-capable TCP implementations MUST NOT set
either ECT codepoint (ECT(0) or ECT(1)) in the IP header for
retransmitted data packets, and that the TCP data receiver SHOULD
ignore the ECN field on arriving data packets that are outside of the
receiver's current window. This is for greater security against
denial-of-service attacks, as well as for robustness of the ECN
congestion indication with packets that are dropped later in the
First, we note that if the TCP sender were to set an ECT codepoint on
a retransmitted packet, then if an unnecessarily-retransmitted packet
was later dropped in the network, the end nodes would never receive
the indication of congestion from the router setting the CE
codepoint. Thus, setting an ECT codepoint on retransmitted data
packets is not consistent with the robust delivery of the congestion
indication even for packets that are later dropped in the network.
In addition, an attacker capable of spoofing the IP source address of
the TCP sender could send data packets with arbitrary sequence
numbers, with the CE codepoint set in the IP header. On receiving
this spoofed data packet, the TCP data receiver would determine that
the data does not lie in the current receive window, and return a
duplicate acknowledgement. We define an out-of-window packet at the
TCP data receiver as a data packet that lies outside the receiver's
current window. On receiving an out-of-window packet, the TCP data
receiver has to decide whether or not to treat the CE codepoint in
the packet header as a valid indication of congestion, and therefore
whether to return ECN-Echo indications to the TCP data sender. If
the TCP data receiver ignored the CE codepoint in an out-of-window
packet, then the TCP data sender would not receive this possibly-
legitimate indication of congestion from the network, resulting in a
violation of end-to-end congestion control. On the other hand, if
the TCP data receiver honors the CE indication in the out-of-window
packet, and reports the indication of congestion to the TCP data
sender, then the malicious node that created the spoofed, out-of-
window packet has successfully "attacked" the TCP connection by
forcing the data sender to unnecessarily reduce (halve) its
congestion window. To prevent such a denial-of-service attack, we
specify that a legitimate TCP data sender MUST NOT set an ECT
codepoint on retransmitted data packets, and that the TCP data
receiver SHOULD ignore the CE codepoint on out-of-window packets.
One drawback of not setting ECT(0) or ECT(1) on retransmitted packets
is that it denies ECN protection for retransmitted packets. However,
for an ECN-capable TCP connection in a fully-ECN-capable environment
with mild congestion, packets should rarely be dropped due to
congestion in the first place, and so instances of retransmitted
packets should rarely arise. If packets are being retransmitted,
then there are already packet losses (from corruption or from
congestion) that ECN has been unable to prevent.
We note that if the router sets the CE codepoint for an ECN-capable
data packet within a TCP connection, then the TCP connection is
guaranteed to receive that indication of congestion, or to receive
some other indication of congestion within the same window of data,
even if this packet is dropped or reordered in the network. We
consider two cases, when the packet is later retransmitted, and when
the packet is not later retransmitted.
In the first case, if the packet is either dropped or delayed, and at
some point retransmitted by the data sender, then the retransmission
is a result of a Fast Retransmit or a Retransmit Timeout for either
that packet or for some prior packet in the same window of data. In
this case, because the data sender already has retransmitted this
packet, we know that the data sender has already responded to an
indication of congestion for some packet within the same window of
data as the original packet. Thus, even if the first transmission of
the packet is dropped in the network, or is delayed, if it had the CE
codepoint set, and is later ignored by the data receiver as an out-
of-window packet, this is not a problem, because the sender has
already responded to an indication of congestion for that window of
In the second case, if the packet is never retransmitted by the data
sender, then this data packet is the only copy of this data received
by the data receiver, and therefore arrives at the data receiver as
an in-window packet, regardless of how much the packet might be
delayed or reordered. In this case, if the CE codepoint is set on
the packet within the network, this will be treated by the data
receiver as a valid indication of congestion.
6.1.6. TCP Window Probes.
When the TCP data receiver advertises a zero window, the TCP data
sender sends window probes to determine if the receiver's window has
increased. Window probe packets do not contain any user data except
for the sequence number, which is a byte. If a window probe packet
is dropped in the network, this loss is not detected by the receiver.
Therefore, the TCP data sender MUST NOT set either an ECT codepoint
or the CWR bit on window probe packets.
However, because window probes use exact sequence numbers, they
cannot be easily spoofed in denial-of-service attacks. Therefore, if
a window probe arrives with the CE codepoint set, then the receiver
SHOULD respond to the ECN indications.
7. Non-compliance by the End Nodes
This section discusses concerns about the vulnerability of ECN to
non-compliant end-nodes (i.e., end nodes that set the ECT codepoint
in transmitted packets but do not respond to received CE packets).
We argue that the addition of ECN to the IP architecture will not
significantly increase the current vulnerability of the architecture
to unresponsive flows.
Even for non-ECN environments, there are serious concerns about the
damage that can be done by non-compliant or unresponsive flows (that
is, flows that do not respond to congestion control indications by
reducing their arrival rate at the congested link). For example, an
end-node could "turn off congestion control" by not reducing its
congestion window in response to packet drops. This is a concern for
the current Internet. It has been argued that routers will have to
deploy mechanisms to detect and differentially treat packets from
non-compliant flows [RFC2309,FF99]. It has also been suggested that
techniques such as end-to-end per-flow scheduling and isolation of
one flow from another, differentiated services, or end-to-end
reservations could remove some of the more damaging effects of
It might seem that dropping packets in itself is an adequate
deterrent for non-compliance, and that the use of ECN removes this
deterrent. We would argue in response that (1) ECN-capable routers
preserve packet-dropping behavior in times of high congestion; and
(2) even in times of high congestion, dropping packets in itself is
not an adequate deterrent for non-compliance.
First, ECN-Capable routers will only mark packets (as opposed to
dropping them) when the packet marking rate is reasonably low. During
periods where the average queue size exceeds an upper threshold, and
therefore the potential packet marking rate would be high, our
recommendation is that routers drop packets rather then set the CE
codepoint in packet headers.
During the periods of low or moderate packet marking rates when ECN
would be deployed, there would be little deterrent effect on
unresponsive flows of dropping rather than marking those packets. For
example, delay-insensitive flows using reliable delivery might have
an incentive to increase rather than to decrease their sending rate
in the presence of dropped packets. Similarly, delay-sensitive flows
using unreliable delivery might increase their use of FEC in response
to an increased packet drop rate, increasing rather than decreasing
their sending rate. For the same reasons, we do not believe that
packet dropping itself is an effective deterrent for non-compliance
even in an environment of high packet drop rates, when all flows are
sharing the same packet drop rate.
Several methods have been proposed to identify and restrict non-
compliant or unresponsive flows. The addition of ECN to the network
environment would not in any way increase the difficulty of designing
and deploying such mechanisms. If anything, the addition of ECN to
the architecture would make the job of identifying unresponsive flows
slightly easier. For example, in an ECN-Capable environment routers
are not limited to information about packets that are dropped or have
the CE codepoint set at that router itself; in such an environment,
routers could also take note of arriving CE packets that indicate
congestion encountered by that packet earlier in the path.
8. Non-compliance in the Network
This section considers the issues when a router is operating,
possibly maliciously, to modify either of the bits in the ECN field.
We note that in IPv4, the IP header is protected from bit errors by a
header checksum; this is not the case in IPv6. Thus for IPv6 the
ECN field can be accidentally modified by bit errors on links or in
routers without being detected by an IP header checksum.
By tampering with the bits in the ECN field, an adversary (or a
broken router) could do one or more of the following: falsely report
congestion, disable ECN-Capability for an individual packet, erase
the ECN congestion indication, or falsely indicate ECN-Capability.
Section 18 systematically examines the various cases by which the ECN
field could be modified. The important criterion considered in
determining the consequences of such modifications is whether it is
likely to lead to poorer behavior in any dimension (throughput,
delay, fairness or functionality) than if a router were to drop a
The first two possible changes, falsely reporting congestion or
disabling ECN-Capability for an individual packet, are no worse than
if the router were to simply drop the packet. From a congestion
control point of view, setting the CE codepoint in the absence of
congestion by a non-compliant router would be no worse than a router
dropping a packet unnecessarily. By "erasing" an ECT codepoint of a
packet that is later dropped in the network, a router's actions could
result in an unnecessary packet drop for that packet later in the
However, as discussed in Section 18, a router that erases the ECN
congestion indication or falsely indicates ECN-Capability could
potentially do more damage to the flow that if it has simply dropped
the packet. A rogue or broken router that "erased" the CE codepoint
in arriving CE packets would prevent that indication of congestion
from reaching downstream receivers. This could result in the failure
of congestion control for that flow and a resulting increase in
congestion in the network, ultimately resulting in subsequent packets
dropped for this flow as the average queue size increased at the
Section 19 considers the potential repercussions of subverting end-
to-end congestion control by either falsely indicating ECN-
Capability, or by erasing the congestion indication in ECN (the CE-
codepoint). We observe in Section 19 that the consequence of
subverting ECN-based congestion control may lead to potential
unfairness, but this is likely to be no worse than the subversion of
either ECN-based or packet-based congestion control by the end nodes.
8.1. Complications Introduced by Split Paths
If a router or other network element has access to all of the packets
of a flow, then that router could do no more damage to a flow by
altering the ECN field than it could by simply dropping all of the
packets from that flow. However, in some cases, a malicious or
broken router might have access to only a subset of the packets from
a flow. The question is as follows: can this router, by altering
the ECN field in this subset of the packets, do more damage to that
flow than if it has simply dropped that set of the packets?
This is also discussed in detail in Section 18, which concludes as
follows: It is true that the adversary that has access only to a
subset of packets in an aggregate might, by subverting ECN-based
congestion control, be able to deny the benefits of ECN to the other
packets in the aggregate. While this is undesirable, this is not a
sufficient concern to result in disabling ECN.
9. Encapsulated Packets
9.1. IP packets encapsulated in IP
The encapsulation of IP packet headers in tunnels is used in many
places, including IPsec and IP in IP [RFC2003]. This section
considers issues related to interactions between ECN and IP tunnels,
and specifies two alternative solutions. This discussion is
complemented by RFC 2983's discussion of interactions between
Differentiated Services and IP tunnels of various forms [RFC 2983],
as Differentiated Services uses the remaining six bits of the IP
header octet that is used by ECN (see Figure 2 in Section 5).
Some IP tunnel modes are based on adding a new "outer" IP header that
encapsulates the original, or "inner" IP header and its associated
packet. In many cases, the new "outer" IP header may be added and
removed at intermediate points along a connection, enabling the
network to establish a tunnel without requiring endpoint
participation. We denote tunnels that specify that the outer header
be discarded at tunnel egress as "simple tunnels".
ECN uses the ECN field in the IP header for signaling between routers
and connection endpoints. ECN interacts with IP tunnels based on the
treatment of the ECN field in the IP header. In simple IP tunnels
the octet containing the ECN field is copied or mapped from the inner
IP header to the outer IP header at IP tunnel ingress, and the outer
header's copy of this field is discarded at IP tunnel egress. If the
outer header were to be simply discarded without taking care to deal
with the ECN field, and an ECN-capable router were to set the CE
(Congestion Experienced) codepoint within a packet in a simple IP
tunnel, this indication would be discarded at tunnel egress, losing
the indication of congestion.
Thus, the use of ECN over simple IP tunnels would result in routers
attempting to use the outer IP header to signal congestion to
endpoints, but those congestion warnings never arriving because the
outer header is discarded at the tunnel egress point. This problem
was encountered with ECN and IPsec in tunnel mode, and RFC 2481
recommended that ECN not be used with the older simple IPsec tunnels
in order to avoid this behavior and its consequences. When ECN
becomes widely deployed, then simple tunnels likely to carry ECN-
capable traffic will have to be changed. If ECN-capable traffic is
carried by a simple tunnel through a congested, ECN-capable router,
this could result in subsequent packets being dropped for this flow
as the average queue size increases at the congested router, as
discussed in Section 8 above.
From a security point of view, the use of ECN in the outer header of
an IP tunnel might raise security concerns because an adversary could
tamper with the ECN information that propagates beyond the tunnel
endpoint. Based on an analysis in Sections 18 and 19 of these
concerns and the resultant risks, our overall approach is to make
support for ECN an option for IP tunnels, so that an IP tunnel can be
specified or configured either to use ECN or not to use ECN in the
outer header of the tunnel. Thus, in environments or tunneling
protocols where the risks of using ECN are judged to outweigh its
benefits, the tunnel can simply not use ECN in the outer header.
Then the only indication of congestion experienced at routers within
the tunnel would be through packet loss.
The result is that there are two viable options for the behavior of
ECN-capable connections over an IP tunnel, including IPsec tunnels:
* A limited-functionality option in which ECN is preserved in the
inner header, but disabled in the outer header. The only
mechanism available for signaling congestion occurring within
the tunnel in this case is dropped packets.
* A full-functionality option that supports ECN in both the inner
and outer headers, and propagates congestion warnings from nodes
within the tunnel to endpoints.
Support for these options requires varying amounts of changes to IP
header processing at tunnel ingress and egress. A small subset of
these changes sufficient to support only the limited-functionality
option would be sufficient to eliminate any incompatibility between
ECN and IP tunnels.
One goal of this document is to give guidance about the tradeoffs
between the limited-functionality and full-functionality options. A
full discussion of the potential effects of an adversary's
modifications of the ECN field is given in Sections 18 and 19.
9.1.1. The Limited-functionality and Full-functionality Options
The limited-functionality option for ECN encapsulation in IP tunnels
is for the not-ECT codepoint to be set in the outside (encapsulating)
header regardless of the value of the ECN field in the inside
(encapsulated) header. With this option, the ECN field in the inner
header is not altered upon de-capsulation. The disadvantage of this
approach is that the flow does not have ECN support for that part of
the path that is using IP tunneling, even if the encapsulated packet
(from the original TCP sender) is ECN-Capable. That is, if the
encapsulated packet arrives at a congested router that is ECN-
capable, and the router can decide to drop or mark the packet as an
indication of congestion to the end nodes, the router will not be
permitted to set the CE codepoint in the packet header, but instead
will have to drop the packet.
The full-functionality option for ECN encapsulation is to copy the
ECN codepoint of the inside header to the outside header on
encapsulation if the inside header is not-ECT or ECT, and to set the
ECN codepoint of the outside header to ECT(0) if the ECN codepoint of
the inside header is CE. On decapsulation, if the CE codepoint is
set on the outside header, then the CE codepoint is also set in the
inner header. Otherwise, the ECN codepoint on the inner header is
left unchanged. That is, for full ECN support the encapsulation and
decapsulation processing involves the following: At tunnel ingress,
the full-functionality option sets the ECN codepoint in the outer
header. If the ECN codepoint in the inner header is not-ECT or ECT,
then it is copied to the ECN codepoint in the outer header. If the
ECN codepoint in the inner header is CE, then the ECN codepoint in
the outer header is set to ECT(0). Upon decapsulation at the tunnel
egress, the full-functionality option sets the CE codepoint in the
inner header if the CE codepoint is set in the outer header.
Otherwise, no change is made to this field of the inner header.
With the full-functionality option, a flow can take advantage of ECN
in those parts of the path that might use IP tunneling. The
disadvantage of the full-functionality option from a security
perspective is that the IP tunnel cannot protect the flow from
certain modifications to the ECN bits in the IP header within the
tunnel. The potential dangers from modifications to the ECN bits in
the IP header are described in detail in Sections 18 and 19.
(1) An IP tunnel MUST modify the handling of the DS field octet at
IP tunnel endpoints by implementing either the limited-
functionality or the full-functionality option.
(2) Optionally, an IP tunnel MAY enable the endpoints of an IP
tunnel to negotiate the choice between the limited-functionality
and the full-functionality option for ECN in the tunnel.
The minimum required to make ECN usable with IP tunnels is the
limited-functionality option, which prevents ECN from being enabled
in the outer header of the tunnel. Full support for ECN requires the
use of the full-functionality option. If there are no optional
mechanisms for the tunnel endpoints to negotiate a choice between the
limited-functionality or full-functionality option, there can be a
pre-existing agreement between the tunnel endpoints about whether to
support the limited-functionality or the full-functionality ECN
All IP tunnels MUST implement the limited-functionality option, and
SHOULD support the full-functionality option.
In addition, it is RECOMMENDED that packets with the CE codepoint in
the outer header be dropped if they arrive at the tunnel egress point
for a tunnel that uses the limited-functionality option, or for a
tunnel that uses the full-functionality option but for which the
not-ECT codepoint is set in the inner header. This is motivated by
backwards compatibility and to ensure that no unauthorized
modifications of the ECN field take place, and is discussed further
in the next Section (9.1.2).
9.1.2. Changes to the ECN Field within an IP Tunnel.
The presence of a copy of the ECN field in the inner header of an IP
tunnel mode packet provides an opportunity for detection of
unauthorized modifications to the ECN field in the outer header.
Comparison of the ECT fields in the inner and outer headers falls
into two categories for implementations that conform to this
* If the IP tunnel uses the full-functionality option, then the
not-ECT codepoint should be set in the outer header if and only
if it is also set in the inner header.
* If the tunnel uses the limited-functionality option, then the
not-ECT codepoint should be set in the outer header.
Receipt of a packet not satisfying the appropriate condition could be
a cause of concern.
Consider the case of an IP tunnel where the tunnel ingress point has
not been updated to this document's requirements, while the tunnel
egress point has been updated to support ECN. In this case, the IP
tunnel is not explicitly configured to support the full-functionality
ECN option. However, the tunnel ingress point is behaving identically
to a tunnel ingress point that supports the full-functionality
option. If packets from an ECN-capable connection use this tunnel,
the ECT codepoint will be set in the outer header at the tunnel
ingress point. Congestion within the tunnel may then result in ECN-
capable routers setting CE in the outer header. Because the tunnel
has not been explicitly configured to support the full-functionality
option, the tunnel egress point expects the not-ECT codepoint to be
set in the outer header. When an ECN-capable tunnel egress point
receives a packet with the ECT or CE codepoint in the outer header,
in a tunnel that has not been configured to support the full-
functionality option, that packet should be processed, according to
whether the CE codepoint was set, as follows. It is RECOMMENDED that
on a tunnel that has not been configured to support the full-
functionality option, packets should be dropped at the egress point
if the CE codepoint is set in the outer header but not in the inner
header, and should be forwarded otherwise.
An IP tunnel cannot provide protection against erasure of congestion
indications based on changing the ECN codepoint from CE to ECT. The
erasure of congestion indications may impact the network and other
flows in ways that would not be possible in the absence of ECN. It
is important to note that erasure of congestion indications can only
be performed to congestion indications placed by nodes within the
tunnel; the copy of the ECN field in the inner header preserves
congestion notifications from nodes upstream of the tunnel ingress
(unless the inner header is also erased). If erasure of congestion
notifications is judged to be a security risk that exceeds the
congestion management benefits of ECN, then tunnels could be
specified or configured to use the limited-functionality option.
9.2. IPsec Tunnels
IPsec supports secure communication over potentially insecure network
components such as intermediate routers. IPsec protocols support two
operating modes, transport mode and tunnel mode, that span a wide
range of security requirements and operating environments. Transport
mode security protocol header(s) are inserted between the IP (IPv4 or
IPv6) header and higher layer protocol headers (e.g., TCP), and hence
transport mode can only be used for end-to-end security on a
connection. IPsec tunnel mode is based on adding a new "outer" IP
header that encapsulates the original, or "inner" IP header and its
associated packet. Tunnel mode security headers are inserted between
these two IP headers. In contrast to transport mode, the new "outer"
IP header and tunnel mode security headers can be added and removed
at intermediate points along a connection, enabling security gateways
to secure vulnerable portions of a connection without requiring
endpoint participation in the security protocols. An important
aspect of tunnel mode security is that in the original specification,
the outer header is discarded at tunnel egress, ensuring that
security threats based on modifying the IP header do not propagate
beyond that tunnel endpoint. Further discussion of IPsec can be
found in [RFC2401].
The IPsec protocol as originally defined in [ESP, AH] required that
the inner header's ECN field not be changed by IPsec decapsulation
processing at a tunnel egress node; this would have ruled out the
possibility of full-functionality mode for ECN. At the same time,
this would ensure that an adversary's modifications to the ECN field
cannot be used to launch theft- or denial-of-service attacks across
an IPsec tunnel endpoint, as any such modifications will be discarded
at the tunnel endpoint.
In principle, permitting the use of ECN functionality in the outer
header of an IPsec tunnel raises security concerns because an
adversary could tamper with the information that propagates beyond
the tunnel endpoint. Based on an analysis (included in Sections 18
and 19) of these concerns and the associated risks, our overall
approach has been to provide configuration support for IPsec changes
to remove the conflict with ECN.
In particular, in tunnel mode the IPsec tunnel MUST support the
limited-functionality option outlined in Section 9.1.1, and SHOULD
support the full-functionality option outlined in Section 9.1.1.
This makes permission to use ECN functionality in the outer header of
an IPsec tunnel a configurable part of the corresponding IPsec
Security Association (SA), so that it can be disabled in situations
where the risks are judged to outweigh the benefits. The result is
that an IPsec security administrator is presented with two
alternatives for the behavior of ECN-capable connections within an
IPsec tunnel, the limited-functionality alternative and full-
functionality alternative described earlier.
In addition, this document specifies how the endpoints of an IPsec
tunnel could negotiate enabling ECN functionality in the outer
headers of that tunnel based on security policy. The ability to
negotiate ECN usage between tunnel endpoints would enable a security
administrator to disable ECN in situations where she believes the
risks (e.g., of lost congestion notifications) outweigh the benefits
The IPsec protocol, as defined in [ESP, AH], does not include the IP
header's ECN field in any of its cryptographic calculations (in the
case of tunnel mode, the outer IP header's ECN field is not
included). Hence modification of the ECN field by a network node has
no effect on IPsec's end-to-end security, because it cannot cause any
IPsec integrity check to fail. As a consequence, IPsec does not
provide any defense against an adversary's modification of the ECN
field (i.e., a man-in-the-middle attack), as the adversary's
modification will also have no effect on IPsec's end-to-end security.
In some environments, the ability to modify the ECN field without
affecting IPsec integrity checks may constitute a covert channel; if
it is necessary to eliminate such a channel or reduce its bandwidth,
then the IPsec tunnel should be run in limited-functionality mode.
9.2.1. Negotiation between Tunnel Endpoints
This section describes the detailed changes to enable usage of ECN
over IPsec tunnels, including the negotiation of ECN support between
tunnel endpoints. This is supported by three changes to IPsec:
* An optional Security Association Database (SAD) field indicating
whether tunnel encapsulation and decapsulation processing allows
or forbids ECN usage in the outer IP header.
* An optional Security Association Attribute that enables
negotiation of this SAD field between the two endpoints of an SA
that supports tunnel mode.
* Changes to tunnel mode encapsulation and decapsulation
processing to allow or forbid ECN usage in the outer IP header
based on the value of the SAD field. When ECN usage is allowed
in the outer IP header, the ECT codepoint is set in the outer
header for ECN-capable connections and congestion notifications
(indicated by the CE codepoint) from such connections are
propagated to the inner header at tunnel egress.
If negotiation of ECN usage is implemented, then the SAD field SHOULD
also be implemented. On the other hand, negotiation of ECN usage is
OPTIONAL in all cases, even for implementations that support the SAD
field. The encapsulation and decapsulation processing changes are
REQUIRED, but MAY be implemented without the other two changes by
assuming that ECN usage is always forbidden. The full-functionality
alternative for ECN usage over IPsec tunnels consists of the SAD
field and the full version of encapsulation and decapsulation
processing changes, with or without the OPTIONAL negotiation support.
The limited-functionality alternative consists of a subset of the
encapsulation and decapsulation changes that always forbids ECN
These changes are covered further in the following three subsections.
22.214.171.124. ECN Tunnel Security Association Database Field
Full ECN functionality adds a new field to the SAD (see [RFC2401]):
ECN Tunnel: allowed or forbidden.
Indicates whether ECN-capable connections using this SA in tunnel
mode are permitted to receive ECN congestion notifications for
congestion occurring within the tunnel. The allowed value enables
ECN congestion notifications. The forbidden value disables such
notifications, causing all congestion to be indicated via dropped
[OPTIONAL. The value of this field SHOULD be assumed to be
"forbidden" in implementations that do not support it.]
If this attribute is implemented, then the SA specification in a
Security Policy Database (SPD) entry MUST support a corresponding
attribute, and this SPD attribute MUST be covered by the SPD
administrative interface (currently described in Section 4.4.1 of
126.96.36.199. ECN Tunnel Security Association Attribute
A new IPsec Security Association Attribute is defined to enable the
support for ECN congestion notifications based on the outer IP header
to be negotiated for IPsec tunnels (see [RFC2407]). This attribute
is OPTIONAL, although implementations that support it SHOULD also
support the SAD field defined in Section 188.8.131.52.
class value type
ECN Tunnel 10 Basic
The IPsec SA Attribute value 10 has been allocated by IANA to
indicate that the ECN Tunnel SA Attribute is being negotiated; the
type of this attribute is Basic (see Section 4.5 of [RFC2407]). The
Class Values are used to conduct the negotiation. See [RFC2407,
RFC2408, RFC2409] for further information including encoding formats
and requirements for negotiating this SA attribute.
Specifies whether ECN functionality is allowed to be used with Tunnel
Encapsulation Mode. This affects tunnel encapsulation and
decapsulation processing - see Section 184.108.40.206.
Values 3-61439 are reserved to IANA. Values 61440-65535 are for
If unspecified, the default shall be assumed to be Forbidden.
ECN Tunnel is a new SA attribute, and hence initiators that use it
can expect to encounter responders that do not understand it, and
therefore reject proposals containing it. For backwards
compatibility with such implementations initiators SHOULD always also
include a proposal without the ECN Tunnel attribute to enable such a
responder to select a transform or proposal that does not contain the
ECN Tunnel attribute. RFC 2407 currently requires responders to
reject all proposals if any proposal contains an unknown attribute;
this requirement is expected to be changed to require a responder not
to select proposals or transforms containing unknown attributes.
220.127.116.11. Changes to IPsec Tunnel Header Processing
For full ECN support, the encapsulation and decapsulation processing
for the IPv4 TOS field and the IPv6 Traffic Class field are changed
from that specified in [RFC2401] to the following:
<-- How Outer Hdr Relates to Inner Hdr -->
Outer Hdr at Inner Hdr at
IPv4 Encapsulator Decapsulator
Header fields: -------------------- ------------
DS Field copied from inner hdr (5) no change
ECN Field constructed (7) constructed (8)
DS Field copied from inner hdr (6) no change
ECN Field constructed (7) constructed (8)
(5)(6) If the packet will immediately enter a domain for which the
DSCP value in the outer header is not appropriate, that value MUST
be mapped to an appropriate value for the domain [RFC 2474]. Also
see [RFC 2475] for further information.
(7) If the value of the ECN Tunnel field in the SAD entry for this
SA is "allowed" and the ECN field in the inner header is set to
any value other than CE, copy this ECN field to the outer header.
If the ECN field in the inner header is set to CE, then set the
ECN field in the outer header to ECT(0).
(8) If the value of the ECN tunnel field in the SAD entry for this
SA is "allowed" and the ECN field in the inner header is set to
ECT(0) or ECT(1) and the ECN field in the outer header is set to
CE, then copy the ECN field from the outer header to the inner
header. Otherwise, make no change to the ECN field in the inner
(5) and (6) are identical to match usage in [RFC2401], although
they are different in [RFC2401].
The above description applies to implementations that support the ECN
Tunnel field in the SAD; such implementations MUST implement this
processing instead of the processing of the IPv4 TOS octet and IPv6
Traffic Class octet defined in [RFC2401]. This constitutes the
full-functionality alternative for ECN usage with IPsec tunnels.
An implementation that does not support the ECN Tunnel field in the
SAD MUST implement this processing by assuming that the value of the
ECN Tunnel field of the SAD is "forbidden" for every SA. In this
case, the processing of the ECN field reduces to:
(7) Set the ECN field to not-ECT in the outer header.
(8) Make no change to the ECN field in the inner header.
This constitutes the limited functionality alternative for ECN usage
with IPsec tunnels.
For backwards compatibility, packets with the CE codepoint set in the
outer header SHOULD be dropped if they arrive on an SA that is using
the limited-functionality option, or that is using the full-
functionality option with the not-ECN codepoint set in the inner
9.2.2. Changes to the ECN Field within an IPsec Tunnel.
If the ECN Field is changed inappropriately within an IPsec tunnel,
and this change is detected at the tunnel egress, then the receipt of
a packet not satisfying the appropriate condition for its SA is an
auditable event. An implementation MAY create audit records with
per-SA counts of incorrect packets over some time period rather than
creating an audit record for each erroneous packet. Any such audit
record SHOULD contain the headers from at least one erroneous packet,
but need not contain the headers from every packet represented by the
9.2.3. Comments for IPsec Support
Substantial comments were received on two areas of this document
during review by the IPsec working group. This section describes
these comments and explains why the proposed changes were not
The first comment indicated that per-node configuration is easier to
implement than per-SA configuration. After serious thought and
despite some initial encouragement of per-node configuration, it no
longer seems to be a good idea. The concern is that as ECN-awareness
is progressively deployed in IPsec, many ECN-aware IPsec
implementations will find themselves communicating with a mixture of
ECN-aware and ECN-unaware IPsec tunnel endpoints. In such an
environment with per-node configuration, the only reasonable thing to
do is forbid ECN usage for all IPsec tunnels, which is not the
In the second area, several reviewers noted that SA negotiation is
complex, and adding to it is non-trivial. One reviewer suggested
using ICMP after tunnel setup as a possible alternative. The
addition to SA negotiation in this document is OPTIONAL and will
remain so; implementers are free to ignore it. The authors believe
that the assurance it provides can be useful in a number of
situations. In practice, if this is not implemented, it can be
deleted at a subsequent stage in the standards process. Extending
ICMP to negotiate ECN after tunnel setup is more complex than
extending SA attribute negotiation. Some tunnels do not permit
traffic to be addressed to the tunnel egress endpoint, hence the ICMP
packet would have to be addressed to somewhere else, scanned for by
the egress endpoint, and discarded there or at its actual
destination. In addition, ICMP delivery is unreliable, and hence
there is a possibility of an ICMP packet being dropped, entailing the
invention of yet another ack/retransmit mechanism. It seems better
simply to specify an OPTIONAL extension to the existing SA
9.3. IP packets encapsulated in non-IP Packet Headers.
A different set of issues are raised, relative to ECN, when IP
packets are encapsulated in tunnels with non-IP packet headers. This
occurs with MPLS [MPLS], GRE [GRE], L2TP [L2TP], and PPTP [PPTP].
For these protocols, there is no conflict with ECN; it is just that
ECN cannot be used within the tunnel unless an ECN codepoint can be
specified for the header of the encapsulating protocol. Earlier work
considered a preliminary proposal for incorporating ECN into MPLS,
and proposals for incorporating ECN into GRE, L2TP, or PPTP will be
considered as the need arises.
10. Issues Raised by Monitoring and Policing Devices
One possibility is that monitoring and policing devices (or more
informally, "penalty boxes") will be installed in the network to
monitor whether best-effort flows are appropriately responding to
congestion, and to preferentially drop packets from flows determined
not to be using adequate end-to-end congestion control procedures.
We recommend that any "penalty box" that detects a flow or an
aggregate of flows that is not responding to end-to-end congestion
control first change from marking to dropping packets from that flow,
before taking any additional action to restrict the bandwidth
available to that flow. Thus, initially, the router may drop packets
in which the router would otherwise would have set the CE codepoint.
This could include dropping those arriving packets for that flow that
are ECN-Capable and that already have the CE codepoint set. In this
way, any congestion indications seen by that router for that flow
will be guaranteed to also be seen by the end nodes, even in the
presence of malicious or broken routers elsewhere in the path. If we
assume that the first action taken at any "penalty box" for an ECN-
capable flow will be to drop packets instead of marking them, then
there is no way that an adversary that subverts ECN-based end-to-end
congestion control can cause a flow to be characterized as being
non-cooperative and placed into a more severe action within the
The monitoring and policing devices that are actually deployed could
fall short of the `ideal' monitoring device described above, in that
the monitoring is applied not to a single flow, but to an aggregate
of flows (e.g., those sharing a single IPsec tunnel). In this case,
the switch from marking to dropping would apply to all of the flows
in that aggregate, denying the benefits of ECN to the other flows in
the aggregate also. At the highest level of aggregation, another
form of the disabling of ECN happens even in the absence of
monitoring and policing devices, when ECN-Capable RED queues switch
from marking to dropping packets as an indication of congestion when
the average queue size has exceeded some threshold.
11. Evaluations of ECN
11.1. Related Work Evaluating ECN
This section discusses some of the related work evaluating the use of
ECN. The ECN Web Page [ECN] has pointers to other papers, as well as
to implementations of ECN.
[Floyd94] considers the advantages and drawbacks of adding ECN to the
TCP/IP architecture. As shown in the simulation-based comparisons,
one advantage of ECN is to avoid unnecessary packet drops for short
or delay-sensitive TCP connections. A second advantage of ECN is in
avoiding some unnecessary retransmit timeouts in TCP. This paper
discusses in detail the integration of ECN into TCP's congestion
control mechanisms. The possible disadvantages of ECN discussed in
the paper are that a non-compliant TCP connection could falsely
advertise itself as ECN-capable, and that a TCP ACK packet carrying
an ECN-Echo message could itself be dropped in the network. The
first of these two issues is discussed in the appendix of this
document, and the second is addressed by the addition of the CWR flag
in the TCP header.
Experimental evaluations of ECN include [RFC2884,K98]. The
conclusions of [K98] and [RFC2884] are that ECN TCP gets moderately
better throughput than non-ECN TCP; that ECN TCP flows are fair
towards non-ECN TCP flows; and that ECN TCP is robust with two-way
traffic (with congestion in both directions) and with multiple
congested gateways. Experiments with many short web transfers show
that, while most of the short connections have similar transfer times
with or without ECN, a small percentage of the short connections have
very long transfer times for the non-ECN experiments as compared to
the ECN experiments.
11.2. A Discussion of the ECN nonce.
The use of two ECT codepoints, ECT(0) and ECT(1), can provide a one-
bit ECN nonce in packet headers [SCWA99]. The primary motivation for
this is the desire to allow mechanisms for the data sender to verify
that network elements are not erasing the CE codepoint, and that data
receivers are properly reporting to the sender the receipt of packets
with the CE codepoint set, as required by the transport protocol.
This section discusses issues of backwards compatibility with IP ECN
implementations in routers conformant with RFC 2481, in which only
one ECT codepoint was defined. We do not believe that the
incremental deployment of ECN implementations that understand the
ECT(1) codepoint will cause significant operational problems. This
is particularly likely to be the case when the deployment of the
ECT(1) codepoint begins with routers, before the ECT(1) codepoint
starts to be used by end-nodes.
11.2.1. The Incremental Deployment of ECT(1) in Routers.
ECN has been an Experimental standard since January 1999, and there
are already implementations of ECN in routers that do not understand
the ECT(1) codepoint. When the use of the ECT(1) codepoint is
standardized for TCP or for other transport protocols, this could
mean that a data sender is using the ECT(1) codepoint, but that this
codepoint is not understood by a congested router on the path.
If allowed by the transport protocol, a data sender would be free not
to make use of ECT(1) at all, and to send all ECN-capable packets
with the codepoint ECT(0). However, if an ECN-capable sender is
using ECT(1), and the congested router on the path did not understand
the ECT(1) codepoint, then the router would end up marking some of
the ECT(0) packets, and dropping some of the ECT(1) packets, as
indications of congestion. Since TCP is required to react to both
marked and dropped packets, this behavior of dropping packets that
could have been marked poses no significant threat to the network,
and is consistent with the overall approach to ECN that allows
routers to determine when and whether to mark packets as they see fit
(see Section 5).