Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
draft-ietf-tsvwg-dscp-considerations-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9435.
|
|
|---|---|---|---|
| Authors | Ana Custura , Gorry Fairhurst , Raffaello Secchi | ||
| Last updated | 2022-06-21 (Latest revision 2022-05-25) | ||
| Replaces | draft-custura-tsvwg-dscp-considerations | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
OPSDIR Last Call review
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by Qin Wu
Has issues
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Wesley Eddy | ||
| IESG | IESG state | Became RFC 9435 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | wesley.m.eddy@gmail.com |
draft-ietf-tsvwg-dscp-considerations-03
TSVWG A. Custura
Internet-Draft G. Fairhurst
Intended status: Informational R. Secchi
Expires: 26 November 2022 University of Aberdeen
25 May 2022
Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
draft-ietf-tsvwg-dscp-considerations-03
Abstract
This document discusses considerations for assigning a new
recommended DiffServ Code Point (DSCP). It considers the common
remarking pathologies that the DiffServ field might be subjected to
along an Internet path. It also notes some implications of using a
specific DSCP.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on 26 November 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Current usage of DSCPs . . . . . . . . . . . . . . . . . . . 4
3.1. IP-Layer Semantics . . . . . . . . . . . . . . . . . . . 4
3.2. Network Control Traffic . . . . . . . . . . . . . . . . . 6
4. Remarking the DSCP . . . . . . . . . . . . . . . . . . . . . 6
4.1. Bleaching the DSCP Field . . . . . . . . . . . . . . . . 7
4.2. IP Type of Service manipulations . . . . . . . . . . . . 8
4.2.1. Impact of ToS Precedence Bleaching . . . . . . . . . 8
4.2.2. Impact of ToS Precedence Remarking . . . . . . . . . 10
4.3. Remarking to a Particular DSCP . . . . . . . . . . . . . 10
5. Interpretation of the IP DSCP at Lower Layers . . . . . . . . 10
5.1. Mapping Specified for IEEE 802 . . . . . . . . . . . . . 11
5.1.1. Mapping Specified for IEEE 802.1 . . . . . . . . . . 11
5.1.2. Mapping Specified for IEEE 802.11 . . . . . . . . . . 11
5.2. DiffServ and MPLS . . . . . . . . . . . . . . . . . . . . 12
5.2.1. Mapping Specified for MPLS . . . . . . . . . . . . . 13
5.2.2. Mapping Specified for MPLS Short Pipe . . . . . . . . 13
5.3. Mapping Specified for Mobile Networks . . . . . . . . . . 14
5.4. Mapping Specified for Carrier Ethernet . . . . . . . . . 15
5.5. Remarking as a Side-effect of Another Policy . . . . . . 15
5.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. Considerations for DSCP Selection . . . . . . . . . . . . . . 16
6.1. Effect of Bleaching . . . . . . . . . . . . . . . . . . . 16
6.2. Where the proposed DSCP > 0x07 (7) . . . . . . . . . . . 16
6.3. Where the proposed DSCP < 0x07 (7) . . . . . . . . . . . 16
6.3.1. Where the proposed DSCP&0x07=0x01 . . . . . . . . . . 16
6.4. Impact on deployed infrastructure . . . . . . . . . . . . 17
6.5. Questions to guide discussion of a proposed new DSCP . . 18
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
The Differentiated Services (DiffServ) architecture has been deployed
in many networks. It provides differentiated traffic forwarding
based on the DiffServ Code Point (DSCP) [RFC2474] carried in the
DiffServ field [RFC2474] of the IP packet header.
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Internet traffic can be associated with a service class, and
categorised into Behavior Aggregates [RFC4594]. In IP networks,
these are associated with a DSCP. Each DSCP is mapped to a Per Hop
Behaviour (PHB). A treatment aggregate is concerned only with the
forwarding treatment of the aggregated traffic, which could be mapped
from a set of DSCP values [RFC5127]. Treatment differentiation can
be realised using a variety of schedulers and queues, and also by
algorithms that implement access to the physical media.
Application -> Service
Traffic Class
|
Behavior -> DiffServ -> Per Hop
Aggregate Codepoint Behavior
|
Treatment -> Schedule
Aggregate Queue, Drop
Figure showing the role of DSCPs in classifying IP traffic for
differential network treatment.
This document discusses considerations for assigning a new DSCP. It
considers some commonly observed DSCP remarking pathologies that
might be experienced along an Internet path. It also describes some
packet forwarding treatments that a packet with a specific DSCP can
expect to receive when forwarded across a link or subnetwork.
The document is motivated by new opportunities to use DiffServ end-
to-end across multiple domains, such as [I-D.ietf-tsvwg-nqb],
proposals to build mechanisms using DSCPs in other standards-setting
organisations, and the desire to use a common set of DSCPs across a
range of infrastructure (e.g., [RFC8622], [I-D.ietf-tsvwg-nqb],
[I-D.learmonth-rfc1226-bis]).
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC2119] [RFC8174] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
DSCPs are specified in the IANA registry [DSCP-registry] where a
variety of different formats are described. A DSCP can sometimes
referred to by name, such as "CS1", and sometimes by a decimal, hex,
or binary value. Hex values will be represented in text using prefix
0x. Binary values will use prefix 0b.
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3. Current usage of DSCPs
This section describes current usage of DSCPs.
3.1. IP-Layer Semantics
The DiffServ architecture specifies use of the DiffServ field in the
IPv4 and IPv6 packet headers to carry one of 64 distinct DSCP values.
Within a given administrative boundary, each DSCP value can be mapped
to a distinct PHB [RFC2474]. When a new PHB is standardized, a
recommended DSCP value among those 64 values is normally reserved for
that PHB, and is assigned by IANA. An operator is not formally
required to use the recommended value; indeed [RFC2474] states that
"the mapping of codepoints to PHBs MUST be configurable." However,
use of the recommended value is usually convenient and avoids
confusion.
The DSCP space is divided into three pools for the purpose of
assignment and management [DSCP-registry]. This use of the pools can
be summarised in a table (where 'x' refers to either a bit position
with value '0' or '1').
DSCP Pool 1: A pool of 32 codepoints with a format 0bxxxxx0, to be
assigned by IANA Standards Action [RFC8126].
DSCP Pool 2: A pool of 16 codepoints with a format 0bxxxx11,
reserved for experimental or local (private) use by network
operators (see sections 4.1 and 4.2 of [RFC8126].
DSCP Pool 3: A pool of 16 codepoints with a 0bxxxx01. This was
initially available for experimental or local use, but which was
originally specified to be preferentially utilised for
standardized assignments if Pool 1 is ever exhausted. Pool 3
codepoints are now utilised for standardized assignments and are
no longer available for experimental or local use [RFC8436].
The DSCP space is shown in the following Figure.
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+---------+-------+----------+-----+----------+-----+----------+-----+
| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 24/CS3 | 25 | 26/AF31 | 27 | 28/AF32 | 29 | 30/AF33 | 31 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 16/CS2 | 17 | 18/AF21 | 19 | 20/AF22 | 21 | 22/AF23 | 23 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 8/CS1 | 9 | 10/AF11 | 11 | 12/AF12 | 13 | 14/AF13 | 15 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+---------+-------+----------+-----+----------+-----+----------+-----+
Figure showing the current list of assigned DSCPs and their assigned
PHBs.
+-----+-----------------------+-----------+
| CS | Class Selector | RFC 2474 |
+-----+-----------------------+-----------+
| BE | Best Effort (CS0) | RFC 2474 |
+-----+-----------------------+-----------+
| AF | Assured Forwarding | RFC 2597 |
+-----+-----------------------+-----------+
| EF | Expedited Forwarding | RFC 3246 |
+-----+-----------------------+-----------+
| VA | Voice Admit | RFC 5865 |
+-----+-----------------------+-----------+
| LE | Lower Effort | RFC 8622 |
+-----+-----------------------+-----------+
Figure showing the summary of the DSCP abbreviations used in previous
RFCs [RFC2474] [RFC2597] [RFC3246] [RFC5865] [RFC8622], as described
in the IANA registry [DSCP-registry]. BE, also known as CS0,
describes the default service.
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The DiffServ architecture allows forwarding treatments to be
associated with each DSCP, and the RFC series describes some of these
as PHBs. Although DSCPs are intended to identify specific treatment
requirements, multiple DSCPs might also be mapped (aggregated) to the
same forwarding treatment. Several IP standards map treatment
aggregates to DSCPs, that might result in remarking: RFC5160
[RFC5160] suggests Meta-QoS-Classes to help enable deployment of
standardized end-to-end QoS classes.
3.2. Network Control Traffic
Network Control Traffic is defined as packet flows that are essential
for stable operation of the administered network (see [RFC4594],
Section 3). This traffic is marked with a value from a set of Class
Selector (CS) DSCPs. This traffic is often a special case within a
provider network, and ingress traffic with these DSCP markings can be
remarked.
DSCP CS2 is recommended for the OAM (Operations, Administration, and
Maintenance) service class (see [RFC4594], Section 3.3).
DSCP CS6 is recommended for local network control traffic. This
includes routing protocols and OAM traffic that are essential to
network operation administration, control and management.
Section 3.2 of RFC4594 [RFC4594] recommends that "CS6 marked packet
flows from untrusted sources (for example, end user devices) SHOULD
be dropped or remarked at ingress to the DiffServ network".
DSCP CS7 is reserved for future use by network control traffic. "CS7
marked packets SHOULD NOT be sent across peering points" [RFC4594].
At the time of writing, there is evidence to suggest CS6 is actively
used by network operators for control traffic. A study of traffic at
a large Internet Exchange showed around 40% of ICMP traffic carried
this mark [IETF113DSCP]. Similarly, another study found many routers
remark all traffic except those packets with a DSCP that sets the
higher order bits to 0b11 (see Section 4 of this document).
4. Remarking the DSCP
It is a feature of the DiffServ architecture that the DiffServ field
of packets can be remarked at domain boundaries (see section 2.3.4.2
of RFC2475 [RFC2475]). A DSCP can be remarked at the ingress of a
DiffServ domain. This remarking can change the DSCP value used on
the remainder of an IP path, or the network can restore the initial
DSCP marking at the egress of the domain. The DiffServ field can
also be remarked based upon common semantics and agreements between
providers at an exchange point. Furthermore, RFC 2475 states that
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remarking must occur when there is a possibility of theft/denial-of-
service attack.
If packets are received that are marked with an unknown or an
unexpected DSCP, RFC2474 [RFC2474] recommends forwarding the packet
using a default (best effort) treatment, but without changing the
DSCP. This seeks to support incremental DiffServ deployment in
existing networks as well as preserve DSCP markings by routers that
have not been configured to support DiffServ. (See also
Section 4.3). RFC3260 [RFC3260] clarifies that this remarking
specified by RFC2474 is intended for interior nodes within a DiffServ
domain. For DiffServ ingress nodes the traffic conditioning required
by RFC 2475 applies first.
Reports measuring existing deployments have categorised DSCP
remarking [Custura] [Barik] into the following seven remarking
pathologies:
Bleach: bleaches all traffic (sets the DSCP to zero);
Bleach-ToS-Precedence: set the first three bits of the DSCP field to
0b000 (reset the 3 bits of the former ToS Precedence field) ;
Bleach-some-ToS: set the first three bits of the DSCP field to 0b000
(reset the 3 bits of the former ToS Precedence field), unless the
first two bits of the DSCP field are 0b11;
Remark-ToS: set the first three bits of the DSCP field to any value
different than 0b000 (replace the 3 bits of the former ToS
Precedence field);
Bleach-low: set the last three bits of the DSCP field to 0b000;
Bleach-some-low: set the last three bits of the DSCP field to 0b000,
unless the first two bits of the DSCP field are 0b11;
Remark: remarks all traffic to one or more particular (non-zero)
DSCP values.
4.1. Bleaching the DSCP Field
A specific form of remarking occurs when the DiffServ field is re-
assigned to the default treatment, CS0 (0x00). This results in
traffic being forwarded using the BE PHB. For example, AF31 (0x1a)
would be bleached to CS0.
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A survey reported that resetting all the bits of the DiffServ field
to 0 was seen to be more prevalent at the edge of the network, and
rather less common in core networks [Custura].
4.2. IP Type of Service manipulations
The IETF first defined ToS precedence for IP packets [RFC0791],
updated by specification as a part of the ToS Field RFC1349
[RFC1349]. Since 1998, this practice has been deprecated by RFC2474
[RFC2474]. RFC 2474 defines DSCPs 0bxxx000 as the Class Selector
codepoints, where PHBs selected by these codepoints MUST meet the
Class Selector PHB Requirements" described in Sec. 4.2.2.2 of that
RFC.
However, a recent survey reports practices based on ToS semantics
have not yet been eliminated from Internet, and need to still be
considered when making new DSCP assignments [Custura].
4.2.1. Impact of ToS Precedence Bleaching
ToS Precedence Bleaching (/Bleach-ToS-Precedence/) is a practice that
resets the first three bits of the DSCP field to zero (the former ToS
Precedence field), leaving the last three bits unchanged (see section
4.2.1 of RFC2474 [RFC2474]). This remarking is distinctive of non-
DiffServ aware routers and a common manipulation of the DiffServ
field. The following Figure provides details.
+-+-+-+-+-+-+
|0 0 0|x x x|
+-+-+-+-+-+-+
Figure showing the ToS Precedence Bleaching (/Bleach-ToS-Precedence/)
pathology, based on Section 3 of RFC1349 [RFC1349]. The bit
positions marked "x" are not changed.
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+---------+-------+----------+-----+----------+-----+----------+-----+
| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 24/CS3 | 25 | 26/AF31 | 27 | 28/AF32 | 29 | 30/AF33 | 31 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 16/CS2 | 17 | 18/AF21 | 19 | 20/AF22 | 21 | 22/AF23 | 23 |
+---------+-------+----------+-----+----------+-----+----------+-----+
| 8/CS1 | 9 | 10/AF11 | 11 | 12/AF12 | 13 | 14/AF13 | 15 |
+=========+=======+==========+=====+==========+=====+==========+=====+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+=========+=======+==========+=====+==========+=====+==========+=====+
As a result of ToS Precedence Bleaching, all the DSCPs in each column
are remarked to the smallest DSCP in that column. The DSCPs in the
bottom row therefore have a higher survivability across an end-to-end
Internet path.
+=========+=======+============+====+======+======+============+====+
| 0/CS0 | 1/LE | 2 | 3 | 4 | 5 | 6 | 7 |
+=========+=======+============+====+======+======+============+====+
|Assigned |ToS Prec Bl.|EXP/|Used |Future|ToS Prec Bl.|Exp/|
| |of AF11..41 |LU |by SSH|NQB |of AF13..EF |LU |
+=================+============+====+======+======+============+====+
Figure showing 0b000xxx DSCPs, highlighting any current assignments
and whether they are affected by any known pathologies. For example,
ToS Precedence Bleaching of popular DSCPs AF11,21,31,41 would result
in traffic being remarked with DSCP 2 in the Internet core. DSCP 4
has been historically used by the SSH application, following
semantics which precede DiffServ[DSCP4].
If ToS Precedence Bleaching occurs, packets with a DSCP 'x' would be
remarked to 'x' & 0x07and then would be treated according to the
corresponding PHB.
A variation of this pathology clears the top three bits of a DSCP,
unless these have values 0b110 or 0b111 (corresponding to the CS6 and
CS7 DSCPs). As a result, a DSCP value greater than 48 decimal (0x30)
is less likely to be impacted by ToS Precedence Bleaching.
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4.2.2. Impact of ToS Precedence Remarking
Practices based on ToS Precedence Remarking (/Remark-ToS/) RFC1349
[RFC1349] were deprecated by RFC2474 [RFC2474]. These practices
based on ToS semantics have not yet been eliminated from deployed
networks.
+-+-+-+-+-+-+
|0 0 1|x x x|
+-+-+-+-+-+-+
Figure showing ToS Precedence Remarking (/Remark-ToS/) pathology,
based on Section 3 of RFC1349 [RFC1349]. The bit positions marked
"x" remain unchanged.
A less common remarking, ToS Precedence Remarking sets the first
three bits of the DSCP to a non-zero value corresponding to a CS PHB.
This remarking is distinctive of non-DiffServ aware routers.
If remarking occurs, packets are forwarded using the PHB specified
for the resulting DSCP. For example, the AF31 DSCP (0x1a) could be
remarked to either AF11 or AF21.
4.3. Remarking to a Particular DSCP
A network device might remark the DiffServ field of an IP packet
based on a local policy with a specific (set of) DSCPs, /Remark/.
This is sometimes performed using a Multi-Field (MF) classifier
[RFC2475] [RFC3290] [RFC4594]. For example, a common remarking is to
remark all traffic to a single DSCP, thus removing any traffic
differentiation (see Section 4.1). Bleaching (/Bleach/) is a
specific example of this pathology that remarks to CS0 (0x00).
In another example, some networks do not follow the recommendation in
RFC2475 [RFC2475], and instead remark packets with an unknown or
unexpected DSCP to the default class, CS0 (0x00) to ensure that
appropriate DSCPs are used within a DiffServ domain. (e.g., see
[RFC8100]).
5. Interpretation of the IP DSCP at Lower Layers
Transmission systems and subnetworks can, and do, utilise the
DiffServ field in an IP packet to select a lower layer forwarding
treatment. In many cases, this use is constrained by designs that
utilise fewer than 6 bits to express the service class, and therefore
infer mapping to a smaller L2 QoS field, for example, WiFi or Multi-
Protocol Label Switching (MPLS).
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5.1. Mapping Specified for IEEE 802
The IEEE specifies standards that include mappings for DSCPs to lower
layer elements.
5.1.1. Mapping Specified for IEEE 802.1
A 3-bit Priority Code Point (PCP) is specified in the IEEE 802.1Q tag
to mark Ethernet frames to one of eight priority values
[IEEE-802-1Q]. The value zero indicates the default best effort
treatment, and the value one indicates a background traffic class.
The remaining values indicate increasing priority. Internet control
traffic can be marked as six, and network control is marked as seven.
The mapping specified in [IEEE-802-1Q] revises a previous standard
[IEEE-802-1D], in an effort to align with DiffServ practice: the
traffic types are specified to match the first three bits of a
suitable DSCP (e.g., the first three bits of the EF DSCP is mapped to
a PCP of 5). However, [IEEE-802-1D] maps both PCP1 (Background) and
PCP2 (Spare) to indicate lower priority than PCP0, RFC8622.
Therefore, different remarking pathologies are expected depending on
the age of deployed devices.
5.1.2. Mapping Specified for IEEE 802.11
Section 6 of RFC 8325 [RFC8325] provides a brief overview of IEEE
802.11 QoS. The IEEE 802.11 standards [IEEE-802-11] provide MAC
functions to support QoS in WLANs using Access Classes (AC). The
upstream User Priority (UP) in the 802.11 header has a 3-bit QoS
value. A DSCP can be mapped to the UP.
Most existing WiFi implementations [RFC8325] use a default mapping
that maps the first three bits of the DSCP to the 802.11 UP value.
This is then in turn mapped to the four WiFi Multimedia (WMM) Access
Categories. The Wi-Fi Alliance has also specified a more flexible
mapping that follows RFC8325 and provides functions at an AP to
remark packets as well as a QoS Map that maps each DSCP to an AC
[WIFI-ALLIANCE].
+-+-+-+-+-+-+
|x x x|. . .|
+-+-+-+-+-+-+
Figure showing the DSCP bits commonly mapped to the UP in 802.11.
The bit positions marked "x" are mapped to the 3-bit UP value, while
the ones marked "." are ignored.
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RFC8325 [RFC8325] notes inconsistencies that can result from such
remarking, and recommends how to perform this remarking. It proposes
several recommendations for mapping a DSCP to an IEEE 802.11 UP for
wired-to-wireless interconnection. The recommendation includes
mapping network control traffic CS6 and CS7, as well unassigned
DSCPs, to UP 0.
In the upstream direction (wireless-to-wired interconnections, this
mapping can result in a specific DSCP remarking pathology. Some
Access Points (APs) currently use a default UP-to-DSCP mapping
RFC8325 [RFC8325], wherein "DSCP values are derived from the layer 2
UP values by multiplying the UP values by eight (i.e., shifting the
three UP bits to the left and adding three additional zeros to
generate a 6-bit DSCP value). This derived DSCP value is used for
QoS treatment between the wireless AP and the nearest classification
and marking policy enforcement point (which may be the centralized
wireless LAN controller, relatively deep within the network).
Alternatively, in the case where there is no other classification and
marking policy enforcement point, then this derived DSCP value will
be used on the remainder of the Internet path." This can result in
remarking /Bleach-low/.
+-+-+-+-+-+-+
|x x x|0 0 0|
+-+-+-+-+-+-+
Figure showing the pathology resulting from deriving from UP-to-DSCP
mapping in some 802.11 networks.
An alternative to UP-to-DSCP remapping uses the DSCP value of a
downstream IP packet (e.g., the Control And Provisioning of Wireless
Access Points (CAPWAP) protocol, RFC5415, maps an IP packet DiffServ
field to the DiffServ field of outer IP header in a CAPWAP tunnel).
Some current constraints of WiFi mapping are discussed in section 2
of [RFC8325]. A QoS profile can be used to limit the maximum DSCP
value used for the upstream and downstream traffic.
5.2. DiffServ and MPLS
Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic
Classes (TCs), which restricts the number of different treatments
[RFC5129]. RFC 5127 describes aggregation of DiffServ TCs [RFC5127],
and introduces four DiffServ Treatment Aggregates. Traffic marked
with multiple DSCPs can be forwarded in a single MPLS TC.
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There are three Label-Switched Router (LSR) behaviors: the Pipe, the
Short Pipe and the Uniform Model [RFC3270]. These only differ when a
LSP performs a push or a pop.
5.2.1. Mapping Specified for MPLS
RFC3270 [RFC3270] defines a flexible solution for support of DiffServ
over MPLS networks. This allows an MPLS network administrator to
select how BAs (marked by DSCPs) are mapped onto Label Switched Paths
(LSPs) to best match the DiffServ, Traffic Engineering and protection
objectives within their particular network.
Mapping from the IP DSCP to the MPLS header are defined in
Section 4.2 of [RFC3270].
The Pipe Model conveys the "LSP Diff-Serv Information" to the LSP
Egress so that its forwarding treatment can be based on the IP DSCP.
When Penultimate Hop Popping (PHP) is used, the Penultimate LSR needs
to be aware of the encapsulation mapping for a PHB to the label
corresponding to the exposed header to perform DiffServ Information
Encoding (Section 2.5.2 of RFC3270 [RFC3270]).
5.2.2. Mapping Specified for MPLS Short Pipe
The Short Pipe Model is an optional variation of the Pipe Model
[RFC3270].
ITU-T Y.1566 [ITU-T-Y-1566] further defined a set of four common QoS
classes and four auxiliary classes to which a DSCP can be mapped when
interconnecting Ethernet, IP and MPLS networks. RFC8100 [RFC8100]
proposes four treatment aggregates for interconnection with four
defined DSCPs. This was motivated by the requirements of MPLS
network operators that use Short-Pipe tunnels, but can be applicable
to other networks, both MPLS and non-MPLS.
RFC8100 recommends preserving the notion of end-to-end service
classes, and recommends that the original DSCP marking is not changed
when treatment aggregates are used. The key requirement is that the
DSCP at the network ingress is restored at the network egress. This
is only feasible in general for a small number of DSCPs. In this
design, packets marked with other DSCPs can be remarked (a /Remark/
pathology) or dealt with via an additional agreement(s) among the
operators of the interconnected networks. Use of the MPLS Short Pipe
Model favours remarking unexpected DSCP values to zero in the absence
of an additional agreement(s), as explained in RFC8100 [RFC8100].
This can result in bleaching (/Bleach/).
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+--------------------------------------+--------+
| RFC8100 | DSCP |
| Agg. Class | |
+--------------------------------------+--------+
|Telephony Service Treatment Aggregate | EF |
| | VA |
+--------------------------------------+--------+
|Bulk Real-Time Treatment Aggregate | AF41 |
| May be added | (AF42) |
| May be added | (AF43) |
+--------------------------------------+--------+
|Assured Elastic Treatment Aggregate | AF31 |
| | AF32 |
| Reserved for the extension of PHBs| (AF33) |
+--------------------------------------+--------+
|Default / Elastic Treatment Aggregate | BE/CS0 |
+--------------------------------------+--------+
|Network Control: Local Use | CS6 |
+--------------------------------------+--------+
The short-pipe MPLS mapping from RFC 8100.
5.3. Mapping Specified for Mobile Networks
Mobile LTE and 5G standards have evolved from older UMTS standards,
and are DiffServ aware. LTE (4G) and 5G standards [SA-5G] identify
traffic classes at the interface between User Equipment (UE) and the
mobile Packet Core network by QCI (QoS Class Identifiers) and 5QI (5G
QoS Identifier). The 3GPP standards do not define or recommend any
specific mapping between each QCI or 5QI and DiffServ (and mobile
QCIs are based on several criteria service class definitions). The
way packets are mapped at the Packet Gateway (P-GW) boundary is
determined by operators. However, TS 23.107 (version 16.0.0, applies
to LTE and below) mandates that Differentiated Services, defined by
IETF, shall be used to interoperate with IP backbone networks.
The GSM Association (GSMA) has defined four aggregated classes and
seven associated PHBs in their guidelines for IPX Provider networks
GSMA IR.34 [GSMA-IR-34]. This was previously specified as the Inter-
Service Provider IP Backbone Guidelines, and provides a mobile ISP to
ISP QoS mapping mechanism, and interconnection with other IP networks
in the general Internet. This describes the case where the DSCP
marking from a Service Provider cannot be trusted (depending on the
agreement between the Service Provider and its IPX Provider),
allowing the IPX Provider to remark the DSCP value to a static
default value.
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+---------------+-------+
| GSMA IR.34 | PHB |
| Agg. Class | |
+---------------+-------+
|Conversational | EF |
+---------------+-------+
| Streaming | AF41 |
+---------------+-------+
| Interactive | AF31 |
+ +-------+
| (ordered by | AF32 |
+ priority, +-------+
| AF3 highest) | AF21 |
+ +-------+
| | AF11 |
+---------------+-------+
| Background | CS0 |
+---------------+-------+
Figure showing the PHB mapping recommended in the guidelines proposed
in GSMA IR.34 [GSMA-IR-34].
5.4. Mapping Specified for Carrier Ethernet
Metro Ethernet Forum (MEF) provides a mapping of DSCPs at the IP
layer to quality of service markings in the Ethernet frame headers
MEF 23.1 [MEF23.1].
5.5. Remarking as a Side-effect of Another Policy
The result of applying a QoS policy (such as matching the IP address,
or traffic reaching a rate limit) could also result in a packet being
remarked to a different DSCP (/Remark/) when it is not admitted to a
service. Traffic marked with an EF and VA DSCP are often policed by
such policies.
Policies and L2 procedures can also result in remarking traffic as a
side-effect of other functions (e.g., in response to DDos).
5.6. Summary
This section has discussed the various ways in which DSCP remarking
pathologies can arise from interactions with lower layers.
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6. Considerations for DSCP Selection
This section provides advice for the assignment of a new DSCP value.
It identifies known issues that might influence the finally assigned
DSCP, followed by a summary of considerations for assignment of a new
DSCP.
6.1. Effect of Bleaching
New DSCP assignments should consider the impact of bleaching, which
can limit the ability to provide the expected treatment end-to-end.
This is particularly important for cases where the codepoint is
intended to result in lower than best effort treatment, as was the
case when defining the LE PHB [RFC8622]. In this case, bleaching, or
remarking to "CS0" would result in elevating the lower effort traffic
to the default class. This is an example of priority inversion.
6.2. Where the proposed DSCP > 0x07 (7)
Although the IETF specifications require systems to use DSCP marking
semantics in place of methods based on the former ToS field, the
current recommendation is that any new assignment where the DSCP is
greater than 0x07 should consider the semantics associated with the
resulting DSCP when ToS Precedence Bleaching is experienced. For
example, it can be desirable to avoid choosing a DSCP that could be
remarked to LE, Lower Effort [RFC8622], which could otherwise
potentially result in a priority inversion in the treatment.
6.3. Where the proposed DSCP < 0x07 (7)
ToS Precedence Bleaching can unintentionally result in extra traffic
aggregated to the same DSCP. For example, after experiencing ToS
Precedence Bleaching, all traffic marked AF11, AF21, AF31 and AF41
would be aggregated with traffic marked with DSCP 2 (0x02),
increasing the volume of traffic which receives the treatment
associated with DSCP 2. New DiffServ Codepoint assignments should
consider unexpected consequences arising from this pathology.
6.3.1. Where the proposed DSCP&0x07=0x01
As a consequence of assigning the LE PHB [RFC8622], the IETF
allocated the DSCP 0b000001 from Pool 3.
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When making assignments where the DSCP has a format: 0bxxx001, the
case of ToS Precedence Bleaching (/Bleach-ToS-Precedence/) of a DSCP
to a value of 0x01 needs to be considered. ToS Precedence Bleaching
will result in demoting the traffic to the lower effort traffic
class. Care should be taken to consider the implications of
remarking when shooing to assign a DSCP with tyhis format.
6.4. Impact on deployed infrastructure
Behaviour of deployed PHBs and conditioning treatments also needs to
be considered when assigning a new DSCP. Network operators have
choices when it comes to configuring DiffServ support within their
domains, and the pathologies described in the previous section can
result from different configurations and approaches:
Networks not remarking DiffServ: A network that implements one or
more PHBs, and does not remark DSCPs. Operators in this category
pass all DSCPs transparently.
Networks that condition the DSCP: A network that implements more
than one PHB and enforces SLAs with its peers. Operators in this
category use conditioning to ensure that only traffic that matches
a policy is permitted to use a specific DSCP (see RFC8100
[RFC8100]). This requires operators to choose to support or
remark a new DSCP assignment.
Networks that remark in some other way , which includes:
1. Networks containing misconfigured devices that do not comply
with the relevant RFCs.
2. Networks containing devices that implement an obsolete
specification or contain software bugs.
3. Networks containing devices that remark the DSCP as a result
of lower layer interactions.
For example, the ToS Precedence Bleaching (/Bleach-ToS-Precedence/)
pathology cannot arise in the case of networks not remarking
DiffServ, but can arise as a result of traffic conditioning at
operator boundaries. It can also arise in the case of
misconfiguration or in networks using old equipment which precedes
DiffServ.
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6.5. Questions to guide discussion of a proposed new DSCP
A series of questions emerge that need to be answered when
considering a proposal to the IETF that requests a new assignment.
These questions include:
* Is the request for local use within a DiffServ domain that does
not require interconnection with other DiffServ domains? This can
use DSCPs in Pool 2 for local or experimental use, without any
IETF specification for the DSCP or associated PHB.
* How is the proposed service class characterised? What are the
characteristics of the traffic to be carried? What are the
expectations for treatment?
* Service Classes that can utilise existing PHBs should use assigned
DSCPs to mark their traffic: Could the request be met by using an
existing IETF DSCP? The proposed new treatment be used to map
traffic to a new or existing PHB.
* Does the service class request assigning new DSCP? Specification
of a new recommended DSCP requires Standards Action. RFC2474
states: "Each standardized PHB MUST have an associated RECOMMENDED
codepoint". If approved, new IETF assignments are normally made
by IANA in Pool 1, but the IETF can request assignments to be made
from Pool 3 [RFC8436].
* Many What are the effects of treatment aggregation on the proposed
method? Section 5.2 describes examples of treatment aggregation.
* What are the implications of using the proposed DSCP on the
expected lower layers over which the traffic may be forwarded?
Section 5 describes some observed treatments by layers below IP.
* If a new DSCP is needed for an end-to-end service, then what is
the expected effect of currently-deployed remarking on the end-to-
end service? Section 4 describes some observed remarking
pathologies, and Section 6.4 identifies root causes for why these
remarking pathologies are observed.
7. Acknowledgments
The authors acknowledge the helpful discussions and analysis by Greg
White and Thomas Fossati in a draft concerning NQB. Ruediger Geib,
and Brian Carpenter contributed comments, along with other memebers
of the TSVWG.
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8. IANA Considerations
This memo provides information to assist in considering new
assignments to the IANA DSCP registry
(https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml).
This memo includes no request to IANA, or update to the IANA
procedures.
9. Security Considerations
The security considerations are discussed in the security
considerations of each cited RFC.
10. References
10.1. Normative References
[DSCP-registry]
IANA, "Differentiated Services Field Codepoints (DSCP)
Registry", https://www.iana.org/assignments/dscp-
registry/dscp-registry.xhtml, 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
<https://www.rfc-editor.org/info/rfc3260>.
[RFC3290] Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
Informal Management Model for Diffserv Routers", RFC 3290,
DOI 10.17487/RFC3290, May 2002,
<https://www.rfc-editor.org/info/rfc3290>.
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[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
2008, <https://www.rfc-editor.org/info/rfc5129>.
[RFC8100] Geib, R., Ed. and D. Black, "Diffserv-Interconnection
Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
March 2017, <https://www.rfc-editor.org/info/rfc8100>.
[RFC8436] Fairhurst, G., "Update to IANA Registration Procedures for
Pool 3 Values in the Differentiated Services Field
Codepoints (DSCP) Registry", RFC 8436,
DOI 10.17487/RFC8436, August 2018,
<https://www.rfc-editor.org/info/rfc8436>.
10.2. Informative References
[Barik] Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
Study", ITC 30, September 2018.
[Custura] Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
modification pathologies in mobile edge networks", TMA ,
2017.
[DSCP4] Kolu, A., "Bogus DSCP value for SSH", online
https://lists.freebsd.org/pipermail/freebsd-
stable/2010-July/057710.html, 2010.
[GSMA-IR-34]
GSM Association, "IR.34 Guidelines for IPX Provider
networks (Previously Inter-Service Provider IP Backbone
Guidelines)", IR 34, 2017.
[I-D.ietf-tsvwg-nqb]
White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
Behavior (NQB PHB) for Differentiated Services", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-nqb-03, 2
November 2020, <http://www.ietf.org/internet-drafts/draft-
ietf-tsvwg-nqb-03.txt>.
[I-D.learmonth-rfc1226-bis]
Learmonth, I., "Internet Protocol Encapsulation of AX.25
Frames", Work in Progress, Internet-Draft, draft-
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learmonth-rfc1226-bis-03, 19 May 2020,
<http://www.ietf.org/internet-drafts/draft-learmonth-
rfc1226-bis-03.txt>.
[IEEE-802-11]
IEEE, "Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) Specifications", IEEE 802.11, 2007.
[IEEE-802-1D]
IEEE, "IEEE Standard for Local and Metropolitan Area
Network-- Media Access Control (MAC) Bridges",
IEEE 802.1D, 2004.
[IEEE-802-1Q]
IEEE, "IEEE Standard for Local and Metropolitan Area
Network-- Bridges and Bridged Networks", IEEE 802.1Q,
2018.
[IETF113DSCP]
Custura, A. and G. Fairhurst, "Considerations for
assigning DSCPs", IETF 113 Proceedings
https://datatracker.ietf.org/meeting/113/materials/slides-
113-tsvwg-61-dscp-considerations-07, 2022.
[ITU-T-Y-1566]
ITU-T, "Quality of Service Mapping and Interconnection
Between Ethernet, Internet Protocol and Multiprotocol
Label Switching Networks", ITU-T Y.1566, 2012.
[MEF23.1] MEF, "MEF Technical Specification MEF 23.1-- Carrier
Ethernet Class of Service ? Phase 2", MEF 23.1, 2012.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC1349] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
<https://www.rfc-editor.org/info/rfc1349>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
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[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
<https://www.rfc-editor.org/info/rfc3270>.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <https://www.rfc-editor.org/info/rfc5127>.
[RFC5160] Levis, P. and M. Boucadair, "Considerations of Provider-
to-Provider Agreements for Internet-Scale Quality of
Service (QoS)", RFC 5160, DOI 10.17487/RFC5160, March
2008, <https://www.rfc-editor.org/info/rfc5160>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8325] Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325, February
2018, <https://www.rfc-editor.org/info/rfc8325>.
[RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
June 2019, <https://www.rfc-editor.org/info/rfc8622>.
[SA-5G] 3GPP, "System Architecture for 5G", TS 23.501, 2019.
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[WIFI-ALLIANCE]
Wi-Fi Alliance, "Wi-Fi QoS Management Specification
Version 2.0", Wi-Fi QoS Management Specification
Version 2.0, 2021.
Appendix A. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
* Individual draft -00, initial document.
* Individual draft -01, address comments from Martin Duke; Brian
Carpenter; Ruediger Geib, and revisions to improve language
consistency.
* Individual draft -02, revise to improve language consistency.
* Working Group -00, replace individual draft.
* Working Group -01, address feedback in preparation for IETF 113
Vienna.
* Working Group -02:
Consolidate terminology after IETF 113 Vienna.
Add clarification to RFC2474 and RFC2475 addressed in RFC3260
(Comments from Ruediger Geib).
Include figures to show the full list of codepoints, their
assigned PHBs & impact of ToS Precedence Bleaching.
Add network categories that differentiate between network
operator approaches to DiffServ.
Add Terminology section to clarify representations of DSCPs.
* Working Group -03
Add table to explain DSCP abbreviations (comment from Brian
Carpenter).
Add some refs, improve language consistency (comments from
Brian Carpenter).
Clarify figure captions.
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Authors' Addresses
Ana Custura
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: ana@erg.abdn.ac.uk
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
Raffaello Secchi
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: r.secchi@abdn.ac.uk
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