Network Working Group Luca Martini
Internet Draft Eric C. Rosen
Expiration Date: August 2006 Cisco Systems, Inc.
Giles Heron
Andrew G. Malis
Tellabs
February 2006
Encapsulation Methods for Transport of PPP/HDLC Over MPLS Networks
draft-ietf-pwe3-hdlc-ppp-encap-mpls-08.txt
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Abstract
A Pseudowire (PW) can be used to carry Point to Point Protocol (PPP),
or High-Level Data Link Control (HDLC) Protocol Data Units over an
Multi Protocol Label Switching (MPLS) network without terminating the
PPP/HDLC protocol. This enables service providers to offer "emulated"
HDLC, or PPP link services over existing MPLS networks. This document
specifies the encapsulation of PPP/HDLC Packet Data Units (PDUs)
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within a pseudo wire.
Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 3
3 Applicability Statement ................................ 5
4 General encapsulation method ........................... 6
4.1 The Control Word ....................................... 6
4.1.1 Setting the sequence number ............................ 8
4.2 MTU Requirements ....................................... 8
5 Protocol-Specific Details .............................. 9
5.1 HDLC ................................................... 9
5.2 Frame Relay Port Mode .................................. 9
5.3 PPP .................................................... 11
6 Using an MPLS Label as the Demultiplexer Field ......... 11
6.1 MPLS Shim EXP Bit Values ............................... 11
6.2 MPLS Shim S Bit Value .................................. 12
7 Congestion Control ..................................... 12
8 IANA Considerations .................................... 12
9 Security Considerations ................................ 13
10 Intellectual Property Statement ........................ 13
11 Full Copyright Statement ............................... 13
12 Normative References ................................... 14
13 Informative References ................................. 14
14 Author Information ..................................... 15
15 Contributing Author Information ........................ 15
1. Specification of Requirements
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 RFC 2119
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2. Introduction
A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
to be carried over an MPLS network. In addressing the issues
associated with carrying a PPP/HDLC PDU over an MPLS network, this
document assumes that a Pseudowire (PW) has been set up by some means
outside the scope of this document. This may be via manual
configuration, or using the signaling protocol such as that defined
in [CONTROL].
The following figure describes the reference models which are derived
from [ARCH] to support the HDLC/PPP PW emulated services. The reader
is also asummmed to be familiar with the content of the [ARCH]
Document.
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +----+ AC V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
| |
native HDLC/PPP service native HDLC/PPP service
Figure 1: PWE3 HDLC/PPP Interface Reference Configuration
This document specifies the emulated PW encapsulation for PPP, and
HDLC, however quality of service related issues are not discussed in
this document. For the purpose of the discussion in this document PE1
will be defined as the ingress router, and PE2 as the egress router.
A layer 2 PDU will be received at PE1, encapsulated at PE1,
transported, decapsulated at PE2, and transmitted out on the
attachment circuit of PE2.
The following reference model describes the termination point of each
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end of the PW within the PE:
+-----------------------------------+
| PE |
+---+ +-+ +-----+ +------+ +------+ +-+
| | |P| | | |PW ter| | PSN | |P|
| |<==|h|<=| NSP |<=|minati|<=|Tunnel|<=|h|<== From PSN
| | |y| | | |on | | | |y|
| C | +-+ +-----+ +------+ +------+ +-+
| E | | |
| | +-+ +-----+ +------+ +------+ +-+
| | |P| | | |PW ter| | PSN | |P|
| |==>|h|=>| NSP |=>|minati|=>|Tunnel|=>|h|==> To PSN
| | |y| | | |on | | | |y|
+---+ +-+ +-----+ +------+ +------+ +-+
| |
+-----------------------------------+
^ ^ ^
| | |
A B C
Figure 2: PW reference diagram
The PW terminates at a logical port within the PE, defined at point B
in the above diagram. This port provides an HDLC Native Service
Processing function that will deliver each PPP/HDLC packet that is
received at point A, unaltered, to the point A in the corresponding
PE at the other end of the PW.
The Native Service Processing (NSP) function includes packet
processing that is required for the PPP/HDLC packets that are
forwarded to the PW termination point. Such functions may include bit
stuffing, PW-PW bridging, L2 encapsulation, shaping, policing, etc.
These functions are specific to the native packet technology , and
may not be required for the PW emulation service.
The points to the left of B, including the physical layer between the
CE and PE, and any adaptation (NSP) functions between it and the PW
terminations, are outside of the scope of PWE3 and are not defined
here.
"PW Termination", between A and B, represents the operations for
setting up and maintaining the PW, and for encapsulating and
decapsulating the PPP/HDLC packets as necessary to transmit them
across the MPLS network.
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3. Applicability Statement
PPP/HDLC transport over PW service is not intended to perfectly
emulate the traditional PPP or HDLC service, but it can be used for
some applications that require PPP or HDLC transport service.
The applicability statements in [FRAME] also apply to the Frame Relay
port mode PW described in this document.
The following are notable differences between traditional PPP/HDLC
service, and the protocol described in this document:
- Packet ordering can be preserved using the OPTIONAL sequence
field in the control word, however implementations are not
required to support this feature.
- The Quality of Service model for traditional PPP/HDLC links can
be emulated, however this is outside the scope of this document.
- A Frame Relay Port mode PW, or HDLC PW, does not process any
packet relay status messages or alarms as described in [Q922]
[Q933]
- The HDLC Flags are processed locally in the PE connected to the
attachment circuit.
The HDLC mode is suitable for port to port transport of Frame Relay
UNI or NNI traffic. Since all packets are passed in a largely
transparent manner over the HDLC PW, any protocol which has HDLC-like
framing may utilize the HDLC PW mode, including PPP, Frame-Relay,
X.25, etc. Exceptions include cases where direct access to the HDLC
interface is required, or modes which operate on the flags, Frame
Check Sequence (FCS) , or bit/byte unstuffing that is performed
before sending the HDLC PDU over the PW. An example of this is PPP
Asynchronous-Control-Character-Map (ACCM) negotiation.
For PPP since media-specific framing is not carried the following
options will not operate correctly if the PPP peers attempt to
negotiate them:
- Frame Check Sequence (FCS) Alternatives
- Address-and-Control-Field-Compression (ACFC)
- Asynchronous-Control-Character-Map (ACCM)
Note also that PW LSP Interface MTU negotiation as specified in
[CONTROL] is not affected by PPP MRU advertisement. Thus if a PPP
peer sends a PDU with a length in excess of that negotiated for the
PW tunnel that PDU will be discarded by the ingress router.
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4. General encapsulation method
This section describes the general encapsulation format for PPP and
HDLC packets over MPLS pseudo wires.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PSN Transport Header (As Required) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudo Wire Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PPP/HDLC Service Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: General format for PPP/HDLC encapsulation over PSNs
The PSN Transport Header depends on the particular tunneling
technology in use. This header is used to transport the encapsulated
PPP/HDLC information through the packet switched core.
The Pseudo Wire Header identifies a particular PPP/HDLC service on a
tunnel. In case of MPLS the Pseudo Wire Header is the MPLS label at
the bottom of the MPLS label stack.
The Control Word is inserted before the PPP/HDLC service payload. It
may contain a length and sequence number.
4.1. The Control Word
There are four requirements that may need to be satisfied when
transporting layer 2 protocols over an MPLS PSN:
-i. Sequentiality may need to be preserved.
-ii. Small packets may need to be padded in order to be
transmitted on a medium where the minimum transport unit is
larger than the actual packet size.
-iii. Control bits carried in the header of the layer 2 packet may
need to be transported.
-iv. Creating an in-band associated channel for operation and
maintenance communications.
The Control Word defined in this section is based on the Generic PW
MPLS Control Word as defined in [CW]. It provides the ability to
sequence individual packets on the PW, avoidance of equal-cost
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multiple-path load-balancing (ECMP) [RFC2992], and enables OAM
mechanisms including [VCCV].
[CW] states, "If a PW is sensitive to packet mis-ordering and is
being carried over an MPLS PSN that uses the contents of the MPLS
payload to select the ECMP path, it MUST employ a mechanism which
prevents packet mis-ordering." This is necessary due to the fact that
ECMP implementations may examine the first nibble after the MPLS
label stack to determine whether the labeled packet is IP or not.
Thus, if the PPP protocol number of an PPP packet carried over the PW
without a control word present begins with 0x4 or 0x6, it could be
mistaken for an IPv4 or IPv6 packet. This could, depending on the
configuration and topology of the MPLS network, lead to a situation
where all packets for a given PW do not follow the same path. This
may increase out-of-order packets on a given PW, or cause OAM packets
to follow a different path than actual traffic.
The features that the control word provides may not be needed for a
given PPP/HDLC PW. For example, ECMP may not be present or active on
a given MPLS network, strict packet sequencing may not be required,
etc. If this is the case, the control word provides little value and
is therefore optional. Early PPP/HDLC PW implementations have been
deployed that do not include a control word or the ability to process
one if present. To aid in backwards compatibility, future
implementations MUST be able to send and receive packets without the
control word present.
In all cases the egress PE MUST be aware of whether the ingress PE
will send a control word over a specific PW. This may be achieved by
configuration of the PEs, or by signaling, as defined in [CONTROL].
The control word is defined as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0|0 0 0 0|Res| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: MPLS PWE3 Control Word
In the above diagram the first 4 bits are set to 0 in indicate a CW
[CW].
The next 4 bits provide space for carrying protocol specific flags.
These are not used for HDLC/PPP and they MUST be set to 0 when
transmitting, and MUST be ignored upon receipt.
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The next 2 bits are reserved for future use, and MUST be ignored.
The next 6 bits provide a length field, which is used as follows: If
the packet's length (defined as the length of the layer 2 payload
plus the length of the control word) is less than 64 bytes, the
length field MUST be set to the packet's length. Otherwise the length
field MUST be set to zero. The value of the length field, if not
zero, is used to remove any padding that may have been added by the
MPLS network. If the control word is used, and padding was added to
the packet while transiting the MPLS network, then when the packet
reaches the egress PE the padding MUST be removed before forwarding
the packet.
The next 16 bits provide a sequence number that can be used to
guarantee ordered packet delivery. The processing of the sequence
number field is OPTIONAL.
The sequence number space is a 16 bit, unsigned circular space. The
sequence number value 0 is used to indicate an unsequenced packet.
4.1.1. Setting the sequence number
The procedures described in section 4 of [CW] MUST be followed to
process the sequence number field.
4.2. MTU Requirements
The network MUST be configured with an MTU that is sufficient to
transport the largest encapsulation packets. When MPLS is used as the
tunneling protocol, for example, this is likely to be 12 or more
bytes greater than the largest packet size. The methodology described
in [FRAG] MAY be used to fragment encapsulated packets that exceed
the PSN MTU. However if [FRAG] is not used then if the ingress router
determines that an encapsulated layer 2 PDU exceeds the MTU of the
PSN tunnel through which it must be sent, the PDU MUST be dropped.
If a packet is received on the attachment circuit that exceeds the
interface MTU subTLV value [CONTROL], it MUST be dropped. It is also
recommended that PPP devices MUST NOT negotiate PPP MRUs larger than
that of the AC MTU.
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5. Protocol-Specific Details
5.1. HDLC
HDLC mode provides port to port transport of HDLC encapsulated
traffic. The HDLC PDU is transported in its entirety, including the
HDLC address, control and protocol fields, but excluding HDLC flags
and the FCS. Bit/Byte stuffing is undone. The control word is
OPTIONAL. If the control word is used then the flag bits in the
control word are not used, and MUST be set to 0 when transmitting,
and MUST be ignored upon receipt.
When the PE detects a status change in the attachment circuit status,
such as an attachment circuit physical link failure, or the AC is
administratively disabled, the PE MUST send the appropriate PW status
notification message that corresponds to the HDLC AC status. In a
similar manner, the local PW status MUST also be reflected in a
respective PW status notification message as described in [CONTROL].
The PW of type 0x0006 "HDLC" will be used to transport HDLC packets.
The IANA allocation registry of "Pseudowire Type" is defined in the
IANA allocation document for PWs [IANA] along with initial allocated
values.
5.2. Frame Relay Port Mode
Figure 5 illustrates the concept of frame relay port mode or many-
to-one mapping which is an OPTIONAL capability.
Figure 5a shows two frame relay devices physically connected with a
frame relay UNI or NNI. Between their two ports P1 and P2, n frame
relay VCs are configured.
Figure 5b shows the replacement of the physical frame relay interface
with a pair of PEs and a PW between them. The interface between a FR
device and a PE is either a FR UNI or NNI. The set of n FR VCs
between the two FR ports P1 and P2 which are controlled by the same
signaling channel using DLCI=0, are mapped into one PW. The standard
frame relay Link Management Interface (LMI) procedures happen
directly between the CEs. Hence with port mode we have many-to-one
mapping between FR VCs and a PW.
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+------+ +-------+
| FR | | FR |
|device| FR UNI/NNI | device|
| [P1]------------------------[P2] |
| | carrying n FR VCs | |
+------+ +-------+
[Pn]: A port
Figure 5a: FR interface between two FR devices
|<---------------------------->|
| |
+----+ +----+
+------+ | | One PW | | +------+
| | | |==================| | | |
| FR | FR | PE1| carrying n FR VCs| PE2| FR | FR |
|device|----------| | | |---------|device|
| CE1 | UNI/NNI | | | | UNI/NNI | CE2 |
+------+ +----+ +----+ +------+
| |
|<----------------------------------------------->|
n FR VCs
Figure 5b: Pseudo-wires replacing the FR interface
FR VCs are not visible individually to a PE; there is no
configuration of individual FR VC in a PE. A PE processes the set of
FR VCs assigned to a port as an aggregate.
FR port mode provides transport between two PEs of a complete FR
frame using the same encapsulation as described above for HDLC mode.
Although frame relay port mode shares the same encapsulation as HDLC
mode, a different PW type is allocated in [IANA]: 0x000F Frame-Relay
Port mode.
All other aspects of this PW type are identical to the HDLC PW
encapsulation described above.
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5.3. PPP
PPP mode provides point to point transport of PPP encapsulated
traffic, as specified in [PPP]. The PPP PDU is transported in its
entirety, including the protocol field (whether compressed using
Protocol Field Compression or not), but excluding any media-specific
framing information, such as HDLC address and control fields or FCS.
If the OPTIONAL control word is used then the flag bits in the
control word are not used, and MUST be set to 0 when transmitting,
and MUST be ignored upon receipt.
When the PE detects a status change in the attachment circuit (AC)
status, such as an attachment circuit physical link failure, or the
AC is administratively disabled, the PE MUST send the appropriate PW
status notification message that corresponds to the PPP AC status. It
should be noted that PPP negotiation status is transparent to the PW,
and MUST NOT be communicated to the remote MPLS PE. In a similar
manner, the local PW status MUST also be reflected in a respective PW
status notification message as described in [CONTROL].
A PW of type 0x0007 "PPP" will be used to transport PPP packets.
The IANA allocation registry of "Pseudowire Type" is defined in the
IANA allocation document for PWs [IANA] along with initial allocated
values.
6. Using an MPLS Label as the Demultiplexer Field
To use an MPLS label as the demultiplexer field, a 32-bit label stack
entry [MPLSENCAP] is simply prepended to the emulated PW
encapsulation, and hence will appear as the bottom label of an MPLS
label stack. This label may be called the "PW label". The particular
emulated PW identified by a particular label value must be agreed by
the ingress and egress LSRs, either by signaling (e.g, via the
methods of [CONTROL]) or by configuration. Other fields of the label
stack entry are set as described below.
6.1. MPLS Shim EXP Bit Values
If it is desired to carry Quality of Service information, the Quality
of Service information SHOULD be represented in the EXP field of the
PW label. If more than one MPLS label is imposed by the ingress LSR,
the EXP field of any labels higher in the stack MUST also carry the
same value.
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6.2. MPLS Shim S Bit Value
The ingress LSR, PE1, MUST set the S bit of the PW label to a value
of 1 to denote that the PW label is at the bottom of the stack.
7. Congestion Control
As explained in [ARCH], the PSN carrying the PW may be subject to
congestion, with congestion characteristics depending on PSN type,
network architecture, configuration, and loading. During congestion
the PSN may exhibit packet loss that will impact the service carried
by the PPP/HLDC PW. In addition, since PPP/HDLC PWs carry an
unspecified type of services across the PSN, they cannot behave in a
TCP-friendly manner prescribed by [RFC2914]. In the presence of
services that reduce transmission rate, PPP/HDLC PWs will thus
consume more than their fair share and SHOULD be halted.
Whenever possible, PPP/HDLC PWs should be run over traffic-engineered
PSNs providing bandwidth allocation and admission control mechanisms.
IntServ-enabled domains providing the Guaranteed Service (GS) or
DiffServ-enabled domains using EF (expedited forwarding) are examples
of traffic-engineered PSNs. Such PSNs will minimize loss and delay
while providing some degree of isolation of the PPP/HDLC PW's effects
from neighboring streams.
The PEs SHOULD monitor for congestion (by using explicit congestion
notification, [VCCV], or by measuring packet loss) in order to ensure
that the service using the PPP/HDLC PW may be maintained. When
significant congestion is detected the PPP/HDLC PW SHOULD be
administratively disabled. If the PW has been set up using the
protocol defined in [CONTROL], then procedures specified in [CONTROL]
for status notification can be used to disable packet transmission on
the ingress PE from the egress PE. The PW may be restarted by manual
intervention, or by automatic means after an appropriate waiting
time.
8. IANA Considerations
This document has no IANA Actions.
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9. Security Considerations
The PPP and HDLC pseudowire type is subject to all of the general
security considerations discussed in [ARCH][CONTROL]. This document
specifies only encapsulations, and not the protocols that may be used
to carry the encapsulated packets across the MPLS network. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein.
10. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
11. Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
12. Normative References
[CONTROL] "Pseudowire Setup and Maintenance using LDP",
Martini, L., et al., draft-ietf-pwe3-control-protocol-16.txt,
( work in progress ), April 2005
[MPLSENCAP] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter,
D. Tappan, G. Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032
[IANA] "IANA Allocations for pseudo Wire Edge to Edge Emulation
(PWE3)" Martini,Townsley, draft-ietf-pwe3-iana-allocation-09.txt
(work in progress), April 2004
[CW] "PWE3 Control Word for use over an MPLS PSN", S. Bryant,
G. Swallow, D. McPherson, draft-ietf-pwe3-cw-06.txt, ( work in
progress ), October 2005.
[PPP] "The Point-to-Point Protocol (PPP)", RFC 1661.
13. Informative References
[FRAME] "Frame Relay over Pseudo-Wires",
draft-ietf-pwe3-frame-relay-06.txt. December 2005,
(work in progress )
[ARCH] "PWE3 Architecture" Bryant, et al.,RFC3985
[FRAG] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley,
draft-ietf-pwe3-fragmentation-08.txt ( work in progress )
February 2005
[VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
Verification (VCCV)", Internet Draft
draft-ietf-pwe3-vccv-08.txt, October 2005. (work in progress)
[RFC2992] RFC-2992: Analysis of an Equal-Cost Multi-Path
Algorithm, C. Hopps, November 2000
[RFC2914] S. Floyd, "Congestion Control Principles" RFC 2914
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14. Author Information
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
e-mail: giles.heron@tellabs.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
e-mail: Andy.Malis@tellabs.com
15. Contributing Author Information
Yeongil Seo
463-1 KT Technology Lab
Jeonmin-dong Yusung-gu
Daegeon, Korea
email: syi1@kt.co.kr
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Toby Smith
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: tob@laurelnetworks.com
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