TRILL (TRansparent Interconnection of Lots of Links): ECN (Explicit Congestion Notification) Support
draft-ietf-trill-ecn-support-07
| Document | Type | Active Internet-Draft (trill WG) | ||
|---|---|---|---|---|
| Authors | Donald Eastlake , Bob Briscoe | |||
| Last updated | 2019-05-09 (latest revision 2018-02-25) | |||
| Replaces | draft-eastlake-trill-ecn-support | |||
| Stream | Internet Engineering Task Force (IETF) | |||
| Intended RFC status | Proposed Standard | |||
| Formats | plain text pdf htmlized bibtex | |||
| Reviews | ||||
| Stream | WG state | Submitted to IESG for Publication | ||
| Document shepherd | Susan Hares | |||
| Shepherd write-up | Show (last changed 2018-01-22) | |||
| IESG | IESG state | RFC Ed Queue | ||
| Action Holders |
(None)
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| Consensus Boilerplate | Yes | |||
| Telechat date | ||||
| Responsible AD | Martin Vigoureux | |||
| Send notices to | "Susan Hares" <shares@ndzh.com> | |||
| IANA | IANA review state | Version Changed - Review Needed | ||
| IANA action state | RFC-Ed-Ack | |||
| RFC Editor | RFC Editor state | MISSREF | ||
| Details | ||||
TRILL Working Group Donald Eastlake
INTERNET-DRAFT Huawei
Intended status: Proposed Standard Bob Briscoe
CableLabs
Expires: August 24, 2018 February 25, 2018
TRILL (TRansparent Interconnection of Lots of Links):
ECN (Explicit Congestion Notification) Support
<draft-ietf-trill-ecn-support-07.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 can improve network
efficiency through better congestion control without packet drops.
This document extends ECN to TRILL (TRansparent Interconnection of
Lots of Links) switches, including integration with IP ECN, and
provides for ECN marking in the TRILL Header Extension Flags Word
(see RFC 7179).
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..................6
3. ECN Support.............................................7
3.1 Ingress ECN Support....................................7
3.2 Transit ECN Support....................................7
3.3 Egress ECN Support.....................................8
3.3.1 Non-ECN Egress RBridges..............................8
3.3.2 ECN Egress RBridges..................................8
4. TRILL Support for ECN Variants.........................11
4.1 Pre-Congestion Notification (PCN).....................11
4.2 Low Latency, Low Loss, Scalable Throughput (L4S)......12
5. IANA Considerations....................................13
6. Security Considerations................................14
7. Acknowledgements.......................................14
Normative References......................................15
Informative References....................................16
Appendix A. TRILL Transit RBridge Behavior to Support L4S.17
Authors' Addresses........................................19
D. Eastlake & B. Briscoe [Page 2]
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1. Introduction
Explicit congestion notification (ECN [RFC3168] [RFC8311]) 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. This can improve network efficiency through better
congestion control without packet drops. The forwarding element can
explicitly mark a proportion of packets in an ECN field instead of
dropping the packet. 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 (TRansparent Interconnection of Lots of Links) 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 [RFC6325].
This document specifies how ECN marking in traffic at the ingress is
copied into the TRILL Extension Header Flags Word and requires such
copying for IP traffic. It also enables congestion marking by a
congested RBridge such as RBn or RB1 above in the TRILL Header
Extension Flags Word [RFC7179].
D. Eastlake & B. Briscoe [Page 3]
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At RB9, the TRILL egress, it specifies how any ECN markings in the
TRILL Header Flags Word and in the encapsulated traffic are combined
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. Such ECN ignorance is not a major problem with
RBridges using this specification because the method specified
assures that, if an egress RBridge is ECN ignorant (so it cannot
further propagate ECN) and congestion has been encountered, the
egress RBridge will at least drop the packet and this drop will
itself indicate congestion to end stations.
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] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
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
D. Eastlake & B. Briscoe [Page 4]
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Not-ECT - Not ECN-Capable Transport
PCN - Pre-Congestion Notification
D. Eastlake & B. Briscoe [Page 5]
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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 32-bit TRILL
Header Extension Flags Word [RFC7780].
These fields are shown 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. The CRItE bit is the critical Ingress-to-Egress summary
bit and will be one if and only if any of the bits in the CItE range
(21-26) are one or there is a critical feature invoked in some
further extension of the TRILL Header after the Extension Flags Word.
The other bits and fields shown in Figure 2 are not relevant to ECN.
See [RFC7780], [RFC7179], and [IANAthFlags] for the meaning of these
other bits and fields.
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 TRILL-ECN and CCE
TRILL Header Extension Flags Word Fields
Table 1 shows the meaning of the codepoints in the TRILL-ECN field.
The first three have the same meaning as the corresponding ECN field
codepoints in the IP header as defined in [RFC3168]. However,
codepoint 0b11 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 6]
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3. ECN Support
This section specifies interworking between TRILL and the original
standardized form of ECN in IP [RFC3168].
The subsections below describe the required behavior to support ECN
at TRILL ingress, transit, and egress. The ingress behavior occurs as
a native frame is encapsulated with a TRILL Header to produce a TRILL
Data packet. The transit behavior occurs in all RBridges where TRILL
Data packets are queued, usually at the output port. The egress
behavior occurs where 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 in
Section 3 and Sections 5.1-5.4 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 behavior at an ingress RBridge 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 set 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 Section 5.3 of
[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, shown 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
D. Eastlake & B. Briscoe [Page 7]
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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.
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 signaling 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
3.3.1 Non-ECN Egress RBridges
If the egress RBridge does not support ECN, that RBridge 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
Section 5.4 of [ECNencapGuide].
3.3.2 ECN Egress RBridges
If an RBridge supports ECN, for the two cases of an IP and a non-IP
inner packet, the egress behavior is as follows:
Decapsulating an inner IP packet: The RBridge sets the ECN field
D. Eastlake & B. Briscoe [Page 8]
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of the outgoing native IP packet using Table 3. 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 2.
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
congestion experienced (CE) but the encapsulated native IP
frame indicates a not ECN-capable transport (Not-ECT), it can
be seen that 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.
Decapsulating a non-IP protocol frame: If the frame has a means of
indicating ECN that is understood by the RBridge, it MUST
follow the guidelines in Section 5.4 of [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 3.
+------------+-----+---------------------+
| 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 2. Mapping of TRILL-ECN and CCE Fields to
the TRILL 3-bit ECN Codepoint Name
D. Eastlake & B. Briscoe [Page 9]
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+---------+----------------------------------------------+
| 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 3. Egress ECN Behavior
An asterisk in the above table indicates a combination that is
currently unused in all variants of ECN marking (see Section 4) and
therefore SHOULD be logged.
With one exception, the mappings in Table 3 are consistent with those
for IP-in-IP tunnels [RFC6040], which ensures backward compatibility
with all current and past variants of ECN marking (see Section 4). It
also ensures forward compatibility with any future form of ECN
marking that complies with the guidelines in [ECNencapGuide],
including cases where ECT(1) represents a second level of marking
severity below CE.
The one exception is that the drop condition in Table 3 need not be
logged because, with TRILL, it is the result of a valid combination
of events.
D. Eastlake & B. Briscoe [Page 10]
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4. TRILL Support for ECN Variants
This section is informative, not normative; it discusses interworking
between TRILL and variants of the standardized form of ECN in IP
[RFC3168]. See also [RFC8311].
The ECN wire protocol for TRILL (Section 2) and the ingress (Section
3.1) and egress (Section 3.3) ECN behaviors have 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 recognized 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. (This difference between ECT(1) and ECT(0) only
applies to PCN, not to the classic ECN support specified for TRILL
in this document before Section 4.)
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 11]
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4.2 Low Latency, Low Loss, Scalable Throughput (L4S)
L4S is currently on 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 12]
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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 13]
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6. Security Considerations
TRILL support of ECN is a straightforward combination of previously
specified ECN and TRILL with no significant new security
considerations.
For ECN tunneling security considerations, see [RFC6040].
For general TRILL protocol security considerations, see [RFC6325].
7. Acknowledgements
The helpful comments of Loa Andersson and Adam Roach are hereby
acknowledged.
This document was prepared with basic NROFF. All macros used were
defined in the source file.
D. Eastlake & B. Briscoe [Page 14]
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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>.
[RFC8174] - Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May
2017, <http://www.rfc-editor.org/info/rfc8174>
[RFC8311] - Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311, DOI
10.17487/RFC8311, January 2018, <https://www.rfc-
editor.org/info/rfc8311>.
[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.
D. Eastlake & B. Briscoe [Page 15]
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Informative References
[ECNL4S] - K. De Schepper, B. Briscoe, "Identifying Modified Explicit
Congestion Notification (ECN) Semantics for Ultra-Low Queueing
Delay", draft-ietf-tsvwg-ecn-l4s-id, work in progress.
[IANAthFlags] - IANA TRILL Extended Header word flags:
http://www.iana.org/assignments/trill-parameters/trill-
parameters.xhtml#extended-header-flags
[RFC6040] - Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November 2010,
<http://www.rfc-editor.org/info/rfc6040>.
[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>.
D. Eastlake & B. Briscoe [Page 16]
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Appendix A. TRILL Transit RBridge Behavior to Support L4S
The specification of the Low Latency, Low Loss, Scalable throughput
(L4S) wire protocol for IP is given in [ECNL4S]. L4S is one example
of the ways TRILL ECN handling may evolve [RFC8311]. It is similar to
the original ECN wire protocol 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.
* bit13 is the least significant bit of the TRILL-ECN field
*/
% On TRILL transit
if (bit13 == 0 ) {
% Classic Queue
if (p > max(random(), random()) )
mark(CCE) % likelihood: p^2
} else {
% L4S Queue
if (p > random() ) {
if (p > 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 17]
INTERNET-DRAFT TRILL ECN Support
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 18]
INTERNET-DRAFT TRILL ECN Support
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
CableLabs
UK
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
D. Eastlake & B. Briscoe [Page 19]
INTERNET-DRAFT TRILL ECN Support
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D. Eastlake & B. Briscoe [Page 20]