Network Working Group Luca Martini
Internet Draft Eric C. Rosen
Expiration Date: September 2004 Cisco Systems, Inc.
Giles Heron Toby Smith
Andrew G. Malis Laurel Networks, Inc.
Tellabs
Yeongil Seo
KT Technology Lab
March 2004
Encapsulation Methods for Transport of PPP/HDLC Over IP and MPLS Networks
draft-ietf-pwe3-hdlc-ppp-encap-mpls-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
A Pseudowire (PW) can be used to carry PPP, or HDLC Protocol Data
Units over an IP or MPLS network without terminating the PPP/HDLC
protocol. This enables service providers to offer "emulated" HDLC, or
PPP link services over existing IP or MPLS networks. This document
specifies the encapsulation of PPP/HDLC PDUs within a pseudowire.
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Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 General encapsulation method ........................... 4
3.1 The Control Word ....................................... 4
3.1.1 Setting the sequence number ............................ 5
3.1.2 Processing the sequence number ......................... 5
3.2 MTU Requirements ....................................... 6
4 Protocol-Specific Details .............................. 6
4.1 HDLC ................................................... 7
4.2 PPP .................................................... 7
5 Using an MPLS Label as the Demultiplexer Field ......... 7
5.1 MPLS Shim EXP Bit Values ............................... 8
5.2 MPLS Shim S Bit Value .................................. 8
6 Security Considerations ................................ 8
7 Intellectual Property Disclaimer ....................... 8
8 References ............................................. 8
9 Author Information ..................................... 9
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
2. Introduction
A PPP/HDLC Pseudowire (PW) allows PPP/HDLC Protocol Data Units (PDUs)
to be carried over an IP network or an MPLS network. In addressing
the issues associated with carrying a PPP/HDLC PDU over a PSN, 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 a signaling protocol such as that defined in [1] or
[7]. As described in [8], this PW may be tunneled through an MPLS,
IPv4 or IPv6 PSN.
The following figure describe the reference models which are derived
from [8] to support the HDLC/PPP PW emulated services.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| PW End V V V V PW End |
V Service +----+ +----+ Service 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. Although different layer 2 protocols require different
information to be carried in this encapsulation, an attempt has been
made to make the encapsulation as common as possible for all layer 2
protocols. Other layer 2 protocols are described in separate
documents. [4] [5] [6]
This document also specifies the way in which the demultiplexer field
is added to the emulated PW encapsulation when an MPLS label is used
as the demultiplexer field. QoS related issues are not discussed in
this document. For the purpose of 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 of PE2.
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3. General encapsulation method
3.1. The Control Word
There are three requirements that may need to be satisfied when
transporting layer 2 protocols over an IP or MPLS backbone:
-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 frame may
need to be transported.
When carrying HDLC/PPP over an IP or MPLS backbone sequentiality may
need to be preserved. The OPTIONAL control word defined here
addresses this requirement. Implementations MUST support sending no
control word, and MAY support sending a control word.
In all cases the egress router must be aware of whether the ingress
router will send a control word over a specific virtual circuit.
This may be achieved by configuration of the routers, or by
signaling, for example as defined in [1]. 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|B E| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: MPLS PWE3 Control Word
In the above diagram the first 4 bits are the PID as defined in [8]
B and E are fragmentation bits and their functionality is specified
in [9].
The next 4 bits provide space for carrying protocol specific flags.
These are not used for HDLC/PPP and they They MUST be set to 0 when
transmitting, and MUST be ignored upon receipt.
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 non-
zero, can be used to remove any padding. When the packet reaches the
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service provider's egress router, it may be desirable to remove the
padding 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.
3.1.1. Setting the sequence number
For a given PW, and a pair of routers PE1 and PE2, if PE1 supports
frame sequencing then the following procedures should be used:
- the initial frame transmitted on the PW MUST use sequence number
1
- subsequent frames MUST increment the sequence number by one for
each frame
- when the transmit sequence number reaches the maximum 16 bit
value (65535) the sequence number MUST wrap to 1
If the transmitting router PE1 does not support sequence number
processing, then the sequence number field in the control word MUST
be set to 0.
3.1.2. Processing the sequence number
If a router PE2 supports receive sequence number processing, then the
following procedures should be used:
When a PW is initially set up, the "expected sequence number"
associated with it MUST be initialized to 1.
When a frame is received on that PW, the sequence number should be
processed as follows:
- if the sequence number on the frame is 0, then the frame passes
the sequence number check
- otherwise if the frame sequence number >= the expected sequence
number and the frame sequence number - the expected sequence
number < 32768, then the frame is in order.
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- otherwise if the frame sequence number < the expected sequence
number and the expected sequence number - the frame sequence
number >= 32768, then the frame is in order.
- otherwise the frame is out of order.
If a packet is in order then, it can be delivered immediately. If the
packet is in order, then the expected sequence number MUST be set
using the algorithm:
expected_sequence_number := frame_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0) then expected_sequence_number := 1;
Packets which are received out of order MAY be dropped or reordered
at the discretion of the receiver.
A simple extension of the above processing algorithm can be used to
detect lost packets.
If a router PE2 does not support receive sequence number processing,
then the sequence number field MAY be ignored.
3.2. MTU Requirements
The network MUST be configured with an MTU that is sufficient to
transport the largest encapsulation frames. If MPLS is used as the
tunneling protocol, for example, this is likely to be 12 or more
bytes greater than the largest frame size. Other tunneling protocols
may have longer headers and require larger MTUs. If the ingress
router determines that an encapsulated layer 2 PDU exceeds the MTU of
the tunnel through which it must be sent, the PDU MUST be dropped. If
an egress router receives an encapsulated layer 2 PDU whose payload
length (i.e., the length of the PDU itself without any of the
encapsulation headers), exceeds the MTU of the destination layer 2
interface, the PDU MUST be dropped.
4. Protocol-Specific Details
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4.1. 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.
The HDLC mode is suitable for port to port transport of Frame Relay
UNI or NNI traffic. It must be noted, however, that this mode is
transparent to the FECN, BECN and DE bits.
4.2. PPP
PPP mode provides point to point transport of PPP encapsulated
traffic, as specified in [3]. The PPP PDU is transported in its
entirety, including the protocol field (whether compressed using PFC
or not), but excluding any media-specific framing information, such
as HDLC address and control fields or FCS. 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 [1]
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.
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.
5. Using an MPLS Label as the Demultiplexer Field
To use an MPLS label as the demultiplexer field, a 32-bit label stack
entry [2] 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 pseudo-
wire identified by a particular label value must be agreed by the
ingress and egress LSRs, either by signaling (e.g, via the methods of
[1]) or by configuration. Other fields of the label stack entry are
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set as follows.
5.1. 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 SHOULD also carry the
same value.
5.2. 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.
6. Security Considerations
This document specifies only encapsulations, and not the protocols
used to carry the encapsulated packets across the network. Each such
protocol may have its own set of security issues, but those issues
are not affected by the encapsulations specified herein.
7. Intellectual Property Disclaimer
This document is being submitted for use in IETF standards
discussions.
8. References
[1] "Transport of Layer 2 Frames Over MPLS",
Martini, L., et al., draft-ietf-pwe3-control-protocol-03.txt,
( work in progress )
[2] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032
[3] "The Point-to-Point Protocol (PPP)", RFC 1661.
[4] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
and MPLS Networks",
draft-ietf-pwe3-atm-encap-02.txt ( work in progress )
[5] "Encapsulation Methods for Transport of Ethernet Frames Over
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IP/MPLS
Networks", draft-ietf-pwe3-ethernet-encap-01.txt. ( work in
progress )
[6] "Frame Relay over Pseudo-Wires", draft-ietf-pwe3-frame-relay-
01.txt. (
work in progress )
[7] "Layer Two Tunneling Protocol (Version 3) "L2TPv3"J. Lau, M.
Townsley,
A. Valencia, G. Zorn, I. Goyret, G. Pall, A. Rubens, B. Palter, ,
work in
progress, draft-ietf-l2tpext-l2tp-base-07.txt, February 2003.
[8] "PWE3 Architecture" Bryant, et al., draft-ietf-pwe3-arch-04.txt (
work
in progress ), June 2003.
[9] "PWE3 Fragmentation and Reassembly", A. Malis,W. M. Townsley,
draft-ietf-pwe3-fragmentation-01.txt ( work in progress ) June
2003
9. 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
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Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
E-mail: erosen@cisco.com
Toby Smith
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: tob@laurelnetworks.com
Andrew G. Malis
Tellabs
90 Rio Robles Dr.
San Jose, CA 95134
e-mail: Andy.Malis@tellabs.com
Yeongil Seo
463-1 KT Technology Lab
Jeonmin-dong Yusung-gu
Daegeon, Korea
email: syi1@kt.co.kr
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