TRILL: ECN (Explicit Congestion Notification) Support
draft-ietf-trill-ecn-support-00
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (trill WG) | |
|---|---|---|---|
| Authors | Donald E. Eastlake 3rd , Bob Briscoe | ||
| Last updated | 2016-10-21 (Latest revision 2016-10-19) | ||
| Replaces | draft-eastlake-trill-ecn-support | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
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| Stream | WG state | WG Document | |
| Document shepherd | Susan Hares | ||
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| Send notices to | "Susan Hares" <shares@ndzh.com> |
draft-ietf-trill-ecn-support-00
TRILL Working Group Donald Eastlake
INTERNET-DRAFT Huawei
Intended status: Proposed Standard Bob Briscoe
Simula Research Lab
Expires: April 18, 2017 October 19, 2016
TRILL: ECN (Explicit Congestion Notification) Support
<draft-ietf-trill-ecn-support-00.txt>
Abstract
Explicit congestion notification (ECN) allows a forwarding element to
notify downstream devices, including the destination, of the onset of
congestion without having to drop packets. This document extends this
capability to TRILL switches, including integration with IP ECN.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Distribution of this document is unlimited. Comments should be sent
to the TRILL working group mailing list <trill@ietf.org>.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
D. Eastlake & B.Briscoe [Page 1]
INTERNET-DRAFT TRILL ECN Support
Table of Contents
1. Introduction............................................3
1.1 Conventions used in this document......................4
2. The ECN Specific Extended Header Flags..................5
3. ECN Support.............................................6
3.1 Ingress ECN Support....................................6
3.2 Transit ECN Support....................................6
3.3 Egress ECN Support.....................................7
4. TRILL Support for ECN Variants..........................9
4.1 Pre-Congestion Notification (PCN)......................9
4.2 Low Latency, Low Loss, Scalable Throughput (L4S)......10
5. IANA Considerations....................................11
6. Security Considerations................................12
7. Acknowledgements.......................................12
Normative References......................................13
Informative References....................................13
Appendix A. TRILL Transit RBridge Behavior to Support L4S.15
Authors' Addresses........................................17
D. Eastlake & B.Briscoe [Page 2]
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1. Introduction
Explicit congestion notification (ECN [RFC3168]) allows a forwarding
element, such as a router, to notify downstream devices, including
the destination, of the onset of congestion without having to drop
packets. Instead, the forwarding element can explicitly mark a
proportion of packets in an ECN field. For example, a two-bit field
is available for ECN marking in IP headers.
.............................
. .
+---------+ .
+------+ | Ingress | .
|Source| +->| RBridge | . +----------+
+---+--+ | | RB1 | . |Forwarding|
| | +------+--+ +----------+ . | Element |
v | . | | Transit | . | Y |
+-------+--+ . +---->| RBridges | . +--------+-+
|Forwarding| . | RBn | . ^ |
| Element | . +-------+--+ +---------+ | v
| X | . | | Egress | | +-----------+
+----------+ . +---->| RBridge +-+ |Destination|
. | RB9 | +-----------+
. TRILL +---------+
. campus .
.............................
Figure 1. Example Path Forwarding Nodes
In [RFC3168] it was recognized that tunnels and lower layer protocols
would need to support ECN, and ECN markings would need to be
propagated, as headers were encapsulated and decapsulated.
[ECNencapGuide] gives guidelines on the addition of ECN to protocols
like TRILL that often encapsulate IP packets, including propagation
of ECN from and to IP.
In the figure above, assuming IP traffic, RB1 is an encapsulator and
RB9 a decapsulator. Traffic from Source to RB1 might or might not get
marked as having experienced congestion in forwarding elements, such
as X, before being encapsulated at ingress RB1. Any such ECN marking
is encapsulated with a TRILL Header.
This specification provides for any ECN marking in the traffic at the
ingress to be copied into the TRILL Extension Header Flags Word. It
also enables congestion marking by a congested RBridge such as RBn or
RB1 above in the TRILL Header Extension Flags Word [RFC7179].
At RB9, the TRILL egress, it specifies how any ECN markings in the
TRILL Header Flags Word and in the encapsulated traffic are combined
D. Eastlake & B.Briscoe [Page 3]
INTERNET-DRAFT TRILL ECN Support
so that subsequent forwarding elements, such as Y and the
Destination, can see if congestion was experienced at any previous
point in the path from Source.
A large part of the guidelines for adding ECN to lower layer
protocols [ECNencapGuide] concerns safe propagation of congestion
notifications in scenarios where some of the nodes do not support or
understand ECN. Whichever RBridges do not support ECN, this
specification ensures congestion notification will propagate safely
to Destination or the packet will be dropped if congestion
notification cannot be propagaed.
1.1 Conventions used in this document
The terminology and acronyms defined in [RFC6325] are used herein
with the same meaning.
In this documents, "IP" refers to both IPv4 and IPv6.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Acronyms:
AQM - Active Queue Management
CCE - Critical Congestion Experienced
CE - Congestion Experienced
CItE - Critical Ingress-to-Egress
ECN - Explicit Congestion Notification
ECT - ECN Capable Transport
L4S - Low Latency, Low Loss, Scalable throughput
NCHbH - Non-Critical Hop-by-Hop
NCCE - Non-Critical Congestion Experienced
Not-ECT - Not ECN-Capable Transport
PCN - Pre-Congestion Notification
D. Eastlake & B.Briscoe [Page 4]
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2. The ECN Specific Extended Header Flags
The extension header fields for explicit congestion notification
(ECN) in TRILL are defined as a two-bit TRILL-ECN field and a one-bit
Critical Congestion Experienced (CCE) field in the TRILL Header
Extension Flags Word [RFC7780].
These fields are show in Figure 2 as "ECN" and "CCE". The TRILL-ECN
field consists of bits 12 and 13, which are in the range reserved for
non-critical hop-by-hop (NCHbH) bits. The CCE field consists of bit
26, which is in the range reserved for Critical Ingress-to-Egress
(CItE) bits. See [RFC7780] and [RFC7179] for the meaning of the other
bits.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Crit.| CHbH | NCHbH |CRSV | NCRSV | CItE | NCItE |
|.....|.........|...........|.....|.......|...........|.........|
|C|C|C| |C|N| | | | | | | | |
|R|R|R| |R|C| |ECN| Ext | | |C|Ext| |
|H|I|R| |C|C| | | Hop | | |C|Clr| |
|b|t|s| |A|A| | | Cnt | | |E| | |
|H|E|v| |F|F| | | | | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 The ECN and CCE TRILL Header Extension Flags Word Fields
Table 1 shows the meaning of the codepoints in the TRILL-ECN field.
Note that the first three have the same meaning as the corresponding
ECN field codepoints in the IPv4 or IPv6 header as defined in
[RFC3168]. However codepoint 11 is called Non-Critical Congestion
Experienced (NCCE) to distinguish it from Congestion Experienced in
IP.
Binary Name Meaning
------ ------- -----------------------------------
00 Not-ECT Not ECN-Capable Transport
01 ECT(1) ECN-Capable Transport (1)
10 ECT(0) ECN-Capable Transport (0)
11 NCCE Non-Critical Congestion Experienced
Table 1. TRILL-ECN Field Codepoints
D. Eastlake & B.Briscoe [Page 5]
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3. ECN Support
The subsections below describe the required behavior to support ECN
at TRILL ingress, transit, and egress. The ingress behavior logically
occurs as a native frame is encapsulated with a TRILL Header to
produce a TRILL Data packet. The transit behavior logically occurs in
all RBridges where TRILL Data packets are queued, usually at the
output port. The egress behavior logically occurs as a TRILL Data
packet is decapsulated and output as a native frame through an
RBridge port.
An RBridge that supports ECN MUST behave as described in the relevant
subsections below, which correspond to the recommended provisions of
[ECNencapGuide]. Nonetheless, the scheme is designed to safely
propagate some form of congestion notification even if some RBridges
in the path followed by a TRILL Data packet support ECN and others do
not.
3.1 Ingress ECN Support
The ingress behavior is as follows:
o When encapsulating an IP frame, the ingress RBridge MUST:
+ set the F flag in the main TRILL header [RFC7780];
+ create a Flags Word as part of the TRILL Header;
+ copy the two ECN bits from the IP header into the TRILL-ECN
field (Flags Word bits 12 and 13)
+ ensure the CCE flag is cleared to zero (Flags Word bit 26).
o When encapsulating a frame for a non-IP protocol, where that
protocol has a means of indicating ECN that is understood by the
ingress RBridge, it MUST follow the guidelines in [ECNencapGuide]
to add a Flags Word to the TRILL Header. For a non-IP protocol
with a similar ECN field to IP, this would be achieved by copying
into the TRILL-ECN field from the encapsulated native frame.
3.2 Transit ECN Support
The transit behavior, show below, is required at all RBridges where
TRILL Data packets are queued, usually at the output port.
o An RBridge that supports ECN MUST implement some form of active
queue management (AQM) according to the guidelines of [RFC7567].
The RBridge detects congestion either by monitoring its own queue
depth or by participating in a link-specific protocol.
D. Eastlake & B.Briscoe [Page 6]
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o If the TRILL Header Flags Word is present, whenever the AQM
algorithm decides to indicate congestion on a TRILL Data packet it
MUST set the CCE flag (Flags Word bit 26).
o If the TRILL header Flags Word is not present, to indicate
congestion the RBridge will either drop the packet or it MAY do
all of the following instead:
+ set the F flag in the main TRILL header;
+ add a Flags Word to the TRILL Header;
+ set the TRILL-ECN field to Not-ECT (00);
+ and set the CCE flag and the Ingress-to-Egress critical summary
bit (CRIbE).
Note that a transit RBridge that supports ECN does not refer to the
TRILL-ECN field before signalling CCE in a packet. It signals CCE
irrespective of whether the packet indicates that the transport is
ECN-capable. The egress/decapsulation behavior (described next)
ensures that a CCE indication is converted to a drop if the transport
is not ECN-capable.
3.3 Egress ECN Support
If the egress RBridge does not support ECN, it will ignore bits 12
and 13 of any Flags Word that is present, because it does not contain
any special ECN logic. Nonetheless, if a transit RBridge has set the
CCE flag, the egress will drop the packet. This is because drop is
the default behavior for an RBridge decapsulating a Critical Ingress-
to-Egress flag when it has no specific logic to understand it. Drop
is the intended behavior for such a packet, as required by
[ECNencapGuide].
If an RBridge supports ECN, the egress behavior is as follows:
o When decapsulating an inner IP packet, the RBridge sets the ECN
field of the outgoing native IP packet using Table 2. It MUST set
the ECN field of the outgoing IP packet to the codepoint at the
intersection of the row for the arriving encapsulated IP packet
and the column for 3-bit ECN codepoint in the arriving outer TRILL
Data packet TRILL Header. If no TRILL Header Extension Flags Word
is present, the 3-bit ECN codepoint is assumed to be all zero
bits.
The name of the TRILL 3-bit ECN codepoint is defined using the
combination of the TRILL-ECN and CCE fields in Table 3.
Specifically, the TRILL 3-bit ECN codepoint is called CE if either
NCCE or CCE is set in the TRILL Header Extension Flags Word.
Otherwise it has the same name as the 2-bit TRILL-ECN codepoint.
In the case where the TRILL 3-bit ECN codepoint indicates
D. Eastlake & B.Briscoe [Page 7]
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congestion experienced (CE) but the encapsulated native IP frame
indicates a not ECN-capable transport (Not-ECT), the RBridge MUST
drop the packet. Such packet dropping is necessary because a
transport above the IP layer that is not ECN-capable will have no
ECN logic, so it will only understand dropped packets as an
indication of congestion.
o When decapsulating a non-IP protocol frame with a means of
indicating ECN that is understood by the RBridge, it MUST follow
the guideines in [ECNencapGuide] when setting the ECN information
in the decapsulated native frame. For a non-IP protocol with a
similar ECN field to IP, this would be achieved by combining the
information in the TRILL Header Flags Word with the encapsulated
non-IP native frame, as specified in Table 2.
+---------+----------------------------------------------+
| Inner | Arriving TRILL 3-bit ECN Codepoint Name |
| Native +---------+------------+------------+----------+
| Header | Not-ECT | ECT(0) | ECT(1) | CE |
+---------+---------+------------+------------+----------+
| Not-ECT | Not-ECT | Not-ECT(*) | Not-ECT(*) | <drop> |
| ECT(0) | ECT(0) | ECT(0) | ECT(1) | CE |
| ECT(1) | ECT(1) | ECT(1)(*) | ECT(1) | CE |
| CE | CE | CE | CE(*) | CE |
+---------+---------+------------+------------+----------+
Table 2: Egress ECN Behavior
An asterisk in the above table indicates a currently unused
combination that SHOULD be logged. In contrast to [RFC6040], in TRILL
the drop condition is the result of a valid combination of events and
need not be logged.
+------------+-----+---------------------+
| TRILL-ECN | CCE | Arriving TRILL 3-bit|
| | | ECN codepoint name |
+------------+-----+---------------------+
| Not-ECT 00 | 0 | Not-ECT |
| ECT(1) 01 | 0 | ECT(1) |
| ECT(0) 10 | 0 | ECT(0) |
| NCCE 11 | 0 | CE |
| Not-ECT 00 | 1 | CE |
| ECT(1) 01 | 1 | CE |
| ECT(0) 10 | 1 | CE |
| NCCE 11 | 1 | CE |
+------------+-----+---------------------+
Table 3: Mapping of TRILL-ECN and CCE Fields to TRILL 3-bit ECN
Codepoint Name
D. Eastlake & B.Briscoe [Page 8]
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4. TRILL Support for ECN Variants
This section is informative, not normative.
Section 3 specifies interworking between TRILL and the original
standardized form of ECN in IP [RFC3168].
The ECN wire protocol for TRILL (Section 2) has been designed to
support the other known variants of ECN, as detailed below. New
variants of ECN will have to comply with the guidelines for defining
alternative ECN semantics [RFC4774]. It is expected that the TRILL
ECN wire protocol is generic enough to support such potential future
variants.
4.1 Pre-Congestion Notification (PCN)
The PCN wire protocol [RFC6660] is recognised by the use of a PCN-
compatible Diffserv codepoint in the IP header and a non-zero IP-ECN
field. For TRILL or any lower layer protocol, equivalent traffic
classification codepoints would have to be defined, but that is
outside the scope of the current document.
The PCN wire protocol is similar to ECN, except it indicates
congestion with two levels of severity. It uses:
o 11 (CE) as the most severe, termed the Excess-traffic-marked (ETM)
codepoint
o 01 ECT(1) as a lesser severity level, termed the Threshold-Marked
(ThM) codepoint.
To implement PCN on a transit RBridge would require a detailed
specification. But in brief:
o the TRILL Critical Congestion Experienced (CCE) flag would be used
for the Excess-Traffic-Marked (ETM) codepoint;
o ECT(1) in the TRILL-ECN field would be used for the Threshold-
Marked codepoint.
Then the ingress and egress behaviors defined in Section 3 would not
need to be altered to ensure support for PCN as well as ECN.
D. Eastlake & B.Briscoe [Page 9]
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4.2 Low Latency, Low Loss, Scalable Throughput (L4S)
L4S is currently only a proposal being considered for adoption onto
the IETF's experimental track. An outline of how a transit TRILL
RBridge would support L4S [ECNL4S] is given in Appendix A.
D. Eastlake & B.Briscoe [Page 10]
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5. IANA Considerations
IANA is requested to update the TRILL Extended Header Flags registry
by replacing the lines for bits 9-13 and for bits 21-26 with the
following:
Bits Purpose Reference
----- ------- ---------
9-11 available non-critical hop-by-hop flags
12-13 TRILL-ECN (Explicit Congestion Notification) [this doc]
21-25 available critical ingress-to-egress flags
26 Critical Congestion Experienced (CCE) [this doc]
D. Eastlake & B.Briscoe [Page 11]
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6. Security Considerations
TRILL support of ECN is a straight forward combination of previously
specified ECN and TRILL with no significnat new security
considerations.
For ECN tunneling security considerations, see [RFC6040].
For general TRILL protocol security considerations, see [RFC6325].
7. Acknowledgements
This document was prepared with basic NROFF. All macros used were
defined in the source file.
D. Eastlake & B.Briscoe [Page 12]
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Normative References
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119,
March 1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC3168] - Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI
10.17487/RFC3168, September 2001, <http://www.rfc-
editor.org/info/rfc3168>.
[RFC4774] - Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124, RFC
4774, DOI 10.17487/RFC4774, November 2006, <http://www.rfc-
editor.org/info/rfc4774>.
[RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
<http://www.rfc-editor.org/info/rfc6325>.
[RFC7179] - Eastlake 3rd, D., Ghanwani, A., Manral, V., Li, Y., and
C. Bestler, "Transparent Interconnection of Lots of Links
(TRILL): Header Extension", RFC 7179, DOI 10.17487/RFC7179, May
2014, <http://www.rfc-editor.org/info/rfc7179>.
[RFC7567] - Baker, F., Ed., and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", BCP 197,
RFC 7567, DOI 10.17487/RFC7567, July 2015, <http://www.rfc-
editor.org/info/rfc7567>.
[RFC7780] - Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
Ghanwani, A., and S. Gupta, "Transparent Interconnection of
Lots of Links (TRILL): Clarifications, Corrections, and
Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
<http://www.rfc-editor.org/info/rfc7780>.
[ECNencapGuide] - B. Briscoe, J. Kaippallimalil, P. Thaler,
"Guidelines for Adding Congestion Notification to Protocols
that Encapsulate IP", draft-ietf-tsvwg-ecn-encap-guidelines,
work in progress.
Informative References
[RFC6040] - Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November 2010,
<http://www.rfc-editor.org/info/rfc6040>.
D. Eastlake & B.Briscoe [Page 13]
INTERNET-DRAFT TRILL ECN Support
[RFC6660] - Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
Pre-Congestion Notification (PCN) States in the IP Header Using
a Single Diffserv Codepoint (DSCP)", RFC 6660, DOI
10.17487/RFC6660, July 2012, <http://www.rfc-
editor.org/info/rfc6660>.
[ECNL4S] - K. De Schepper, B. Briscoe, I. Tsang, "Identifying
Modified Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queueing Delay", draft-briscoe-tsvwg-ecn-l4s-id, work
in progress.
D. Eastlake & B.Briscoe [Page 14]
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Appendix A. TRILL Transit RBridge Behavior to Support L4S
An initial specification of the Low Latency, Low Loss, Scalable
throughput (L4S) wire protocol for IP is given in [ECNL4S]. It is
similar to the original ECN wire protoocl for IP [RFC3168], except:
o An AQM that supports L4S classifies packets with ECT(1) or CE in
the IP header into an L4S queue and a "Classic" queue otherwise.
o the meaning of CE markings applied by an L4S queue is not the same
as the meaning of a drop by a "Classic" queue (contrary to the
original requirement for ECN [RFC3168]). Instead the likelihood
that the Classic queue drops packets is defined as the square of
the likelihood that the L4S queue marks packets (e.g. when there
is a drop probability of 0.0009 (0.09%) the L4S marking
probability will be 0.03 (3%)).
This seems to present a problem for the way that a transit TRILL
RBridge defers the choice between marking and dropping to the egress.
Nonetheless, the following pseudocode outlines how a transit TRILL
RBridge can implement L4S marking in such a way that the egress
behavior already described in Section 3.3 for Classic ECN [RFC3168]
will produce the desired outcome.
/* p is an internal variable calculated by any L4S AQM
* dependent on the delay being experienced in the Classic queue.
* bit23 is the least significant bit of the TRILL-ECN field
*/
% On TRILL transit
if (bit23 == 0 ) {
% Classic Queue
if (p > max(random(), random()) )
mark(CCE) % likelihood: p^2
} else {
% L4S Queue
if (p > max(random()) ) {
if (p > max(random()) )
mark(CCE) % likelihood: p^2
else
mark(NCCE) % likelihood: p - p^2
}
}
With the above transit behavior, an egress that supports ECN (Section
3.3) will drop packets or propagate their ECN markings depending on
whether the arriving inner header is from a non-ECN-capable or ECN-
capable transport.
D. Eastlake & B.Briscoe [Page 15]
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Even if an egress has no L4S-specific logic of its own, it will drop
packets with the square of the probability that an egress would if it
did support ECN, for the following reasons:
o Egress with ECN support:
+ L4S: propagates both the Critical and Non-Critical CE marks
(CCE & NCCE) as a CE mark.
Likelihood: p^2 + p - p^2 = p
+ Classic: Propagates CCE marks as CE or drop, depending on
inner.
Likelihood: p^2
o Egress without ECN support:
+ L4S: does not propagate NCCE as a CE mark, but drops CCE marks.
Likelihood: p^2
+ Classic: drops CCE marks.
Likelihood: p^2
D. Eastlake & B.Briscoe [Page 16]
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Authors' Addresses
Donald E. Eastlake, 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
Tel: +1-508-333-2270
Email: d3e3e3@gmail.com
Bob Briscoe (editor)
Simula Research Lab
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
D. Eastlake & B.Briscoe [Page 17]
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D. Eastlake & B.Briscoe [Page 18]