MPLS Working Group Liwen Wu, Pierrick Cheval
Internet Draft Alcatel USA
Expiration Date: May 1999
Pasi Vaananen
Nokia
November 1998
MPLS Extensions for Differential Services
draft-wu-mpls-diff-ext-00.txt
0. Status of this Memo
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1. Abstract
This document provides new message formats and the updates of the
current MPLS LDP messages and MPLS RSVP messages to support the
differentiated services (DiffServ).
We first discuss the extension to the current MPLS architecture
[MPLSARCH] to support differentiated services [DIFFARCH] over MPLS
cloud.
For a Behavior Aggregate (BA), we define "Behavior Aggregate
Selector (BAS)" which related to the forwarding queue behavior, and
define "Behavior Modifier (BM)" which related to the dropping
behavior. Each DiffServ Per Hop Behavior (PHB) can be represented
using either BAS only or combination of BAS and BM.
In brief, we propose that all LSRs on the path of a single MPLS LSP
apply the same Behavior Aggregate Selector (BAS) for the traffic
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carried in this LSP, on the basis of behavior class selector that is
specified as part of LSP set-up process. Behavior Modifier (BM),
carried as part of label encapsulation header, is used to further
modify the behavior selected by BAS. Behavior modifiers are carried
as part of MPLS shim header, ATM cell header or Frame Relay header,
and can be used to indicate, e.g. drop priority or congestion
indication inside a single BAS.
This proposal is not intended to prevent any potential future
evolution that would accomodate multiple, different Behavior
Aggregate Selectors (BAS) within a single LSP.
2. Introduction
In an MPLS domain, when a stream of data traverses a same path, an
Label Switch Path (LSP) can be established through MPLS signalling
protocols. At the ingress Label Switch Router (LSR), the packet is
assigned with a label and is transmitted downstream. At each LSR
along the LSP, the label is used to forward the packet to the next
hop.
In a DiffServ domain, when a stream of data have a same forwarding
behavior, packets belong to that stream are assigned with the same
behavioral aggregate. At the ingress node the packet is classified
and assigned with a Differentiated Services Code Point (DSCP). which
represents a class of service. At each transit node, the destination
address is used to decide the next hop. The DSCP is used to select
the Per Hop Behaviour (PHB) that determines the queuing treatment
and, in some cases, drop propability inside the queue for each
packet.
To combine the MPLS and Diffserv together, when a stream of data
traverse the same path and is forwarded using same PHB, an LSP can be
established, leading to selection of same queuing treatment in LSRs
along the path.
Each PHB is then mapped to the established BAS, or the combination of
BAS (determined by label) and BM field, when drop priority or other
treatment that modifies the forwarding treatment inside the queue is
specified.
The method and consequences of building an LSP which can support
multiple BAS are for further study (FFS).
2.1 The Next Hop Label Forwarding Entry (NHLFE)
As described in [MPLSARCH] document, The "Next Hop Label Forwarding
Entry" (NHLFE) is used when forwarding a labeled packet. It contains
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the following information:
1. the packet's next hop
2. the data link encapsulation to use when transmitting the packet
3. the way to encode the label stack when transmitting the packet
4. the operation to perform on the packet's label stack
To combine MPLS and Diffserv, the NHLFE will be extended to include:
5. the packet's behavior aggregate selector, which determines the
queuing treatment of the packet carried on this LSP.
2.2 Setting Up an LSP Which Support Differentiated Services
As described in [MPLSARCH], an LSP can be established through
"downstream-on-demand" or "downstream" label distribution.
"Downstream-on-demand" means an upstream LSR explicitly requests from
its next hop for a particular FEC, a label binding for that FEC,
whereas "downstream" means an downstream LSR to distribute bindings
to upstream LSRs that have not explicitly requested them.
To support DiffServ, the Diffserv related behavioral aggregate
attributes will be added to the MPLS label request messages and MPLS
label binding messages. The detailed format of these messages will be
explained later. The MPLS control application on each LSR along the
LSP will process the new attributes and install the DiffServ
forwarding behavior for that LSP.
2.3 Issues Related to Label-Merge
In an MPLS domain, two or more LSPs can be merged into one LSP at one
LSR. Here LSPs which carry certain Differentiated Service traffic
can only be merged into one LSP, if they carry the traffic which
belongs to the same behavioral aggregate.
As of today, in differentiated service domain, there are 2 kinds of
PHBs defined:
-- EF PHB [DIFF_EF]
The EF traffic is forwarded with low loss, low latency, low jitter.
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The aggregate arrival rate that is associated with same egress
interface in the node is always less than that interfaces
configured minimum departure rate for EF PHB.
Since EF traffic requires absolute bandwidth, when 2 EF traffic
LSPs are merged using downstream-on-demand ordered mode, it is
recommended the "brute force" method is used. The "brute force"
mode means that at the merge point an updated label request message
is sent all the way from the merge point to the egress LSR. Each
LSR along the LSP must make sure the incoming EF traffic rate is
less than the interface's configured EF minimum departure rate.
-- AF PHB [DIFF_AF]
As described in [DIFF_AF], Assured Forwarding (AF) PHB group
provides forwarding of IP packets in N independent AF classes.
Within each AF class, an IP packet is assigned one of M different
levels of drop precedence. An IP packet that belongs to an AF class
i and has drop precedence j is marked with the AF codepoint AFij,
where 1 <= i <= N and 1 <= j <= M. Currently, four classes (N=4)
with three levels of drop precedence in each class (M=3) are
defined for general use.
Two LSPs are allowed to be merged into one LSP only when 2 LSPs
carry the traffic which has the same AF class.
Since AF class traffic requires relative bandwidth, when 2 LSPs
merged there is no need to send a new label request all the way
down the egress node.
The subject of AF traffic parameters is for furthur study (FFS).
2.4 Data Transfer in an MPLS LSP
2.4.1 Shim Header Encapsulation
When MPLS shim header is used, the traffic carried through an MPLS
LSP is wrapped with at least one MPLS shim header. In the shim
header, there is a field currently specified as "Experimental Use".
We propose to use this field to represent BM field, and specifically
drop precedence level for AF traffic.
Mapping from AF codepoints AFij to the BM field are as follows:
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AF Class BM Fields:
AFx1 ------------> 001
AFx2 ------------> 010
AFx3 ------------> 011
Traffic from more than one AF class AFij (where i is not equal ) MUST
NOT be mapped onto same LSP.
2.4.2. ATM header encapsulation
AF Class: CLP bit:
AFx1 --> 0
AFx2 --> 1
AFx3 --> 1
AF aggregate class selection for ATM is performed by mapping the AFxj
to desired ATM LSP, with appropriate traffic class and traffic
parameters. ATM traffic class selection and parameter mapping is not
discussed in this version of the document.
2.4.3. Frame Relay header encapsulation
AF Class: DE bit:
AFx1 --> 0
AFx2 --> 1
AFx3 --> 1
AF aggregate class selection for FR is performed by mapping the AFxj
to desired FR LSP, with appropriate traffic traffic parameters.
Frame relay parameter mapping is not discussed in this version of the
document.
3. Message Format
As of today, there are two signalling protocols proposed by MPLS
working group: RSVP [MPLS_RSVP] and LDP [MPLS_LDP]. The following
sections proposes the new messages and new objects for differentiated
service support.
3.1 New RSVP Object Format
The RSVP SENDER_TSPEC object is expanded to carry the information
which PHB is used to forward the traffic.
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3.1.1 SENDER_TSPEC Object for EF PHB
DiffServ EF PHB, SENDER_TSPEC object: Class = 12, C-Type = 3
The format can be
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (bytes) | Class-Num | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| The Committed Data Rate |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-- The committed Data Rate, the peak rate that traffic is allowed
to enter into the network.
3.1.2 SENDER_TSPEC Object for AF PHB
DiffServ AF PHB, SENDER_TSPEC object: Class = 12, C-Type = 4
The format can be
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (bytes) | Class-Num | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| The AF Class Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-- The AF Class Number, which contains the AF class number for the
traffic
3.2 New LDP Object Format
3.2.1 The New COS TLV for EF and AF PHBs
COS TLV is expanded to contain the following values:
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COS Name Type code Value
EF PHB 0x02 4 octet which is the Committed Data Rate
(CDR)
AF PHB 0x03 1 octet, which can be class1, class2,
class3, class4
3.2.2 The New LDP Label Re-request Message Format
A new message, Label Re-request, is defined for renegotiating the
bandwidth requirement of an established LSP. When 2 LSPs which carry
the EF traffic merged at one LSR, an Label Re-Request message can be
sent down from the merged point to the egress LSR. This Label Re-
Request carries the aggregated Committed Data Rate of the 2 LSPs.
The format of this Label Re-request is very similar to the Label
Request message [MPLS_LDP].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Re-Request (0x0406) | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC-Request TLV 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC-Request TLV n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Id
Four octet integer used to identify this message.
FEC-Request TLV
Each specifies an FEC for which a label mapping is requested. A
FEC-Request TLV is a nested TLV that contains a FEC TLV, an
optional COS TLV, and an optional Hop Count TLV.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC-Request (0x0701) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| COS TLV (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hop Count TLV (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The encoding for the FEC, COS, and Hop Count TLVs are specified in
Section "Commonly Used TLVs".
Optional Parameters
No optional parameters are defined for the Label Request message.
4. Security
The security issues will be discussed in the future version of this
document.
5. Achnowledgments
Thanks to Michel Henrion, Emmanuel Desmet, Bernard Sales, Christophe
Boscher, Olivier Duroyon and Sudheer Dharanikota for their helpful
review and feedback.
6. References
[MPLSARCH] Rosen et al, "Multiprotocol Label Switching Architecture",
working in progress, (draft-ietf-mpls-arch-02), July, 1998
[DIFFARCH] Blake et al, "An Architecture for Differentiated
Services", working in progress, (draft-ietf-diffserv-arch-02.txt),
October, 1998
[DIFF_AF] Heinanen et al, "Assured Forwarding PHB Group", working in
progress, (draft-ietf-diffserv-af-02.txt), October, 1998
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[DIFF_EF] Jacobson et al, "An Expedited Forwarding PHB", working in
progress, (draft-ietf-diffserv-phb-ef-00.txt), August, 1998
[MPLS_LDP] Andersson et al, "LDP Specification", working in progress,
(draft-ietf-mpls-ldp-00.txt), August, 1998
[MPLS_RSVP] Swallow et al, "Extensions to RSVP for Traffic
Engineering", working in progress, (draft-swallow-mpls-rsvp-trafeng-
00.txt), Auguest, 1998
7. Author Information
Liwen Wu
Email: liwen.wu@adn.alcatel.com Tel: 703-724-2619
44983 Knoll Square, Ashburn, VA 20147, USA
Pierrick Cheval
Email: pierrick.cheval@adn.alcatel.com Tel: 703-724-2080
44983 Knoll Square, Ashburn, VA 20147, USA
Pasi Vannanen
Email: pasi.vaananen@ntc.nokia.com Tel:781-238-4981
3 Burlington Woods Drive, Suite 250, Burlington, MA. 01803
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