PPP/HDLC over MPLS: Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks
Carrying PPP and HDLC links across an MPLS network as pseudowires, without ending the link protocol.
Martini, L., Rosen, E., Heron, G., and A. Malis, "Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks", RFC 4618, DOI 10.17487/RFC4618, September 2006, <https://www.rfc-editor.org/info/rfc4618>.
@misc{rfc4618,
series = {Request for Comments},
number = 4618,
howpublished = {RFC 4618},
publisher = {RFC Editor},
doi = {10.17487/RFC4618},
url = {https://www.rfc-editor.org/info/rfc4618},
author = {L. Martini and E. Rosen and G. Heron and A. Malis},
title = {{Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks}},
pagetotal = 16,
year = 2006,
month = sep,
} RFC text
Contents
Network Working Group L. Martini
Request for Comments: 4618 E. Rosen
Category: Standards Track Cisco Systems, Inc.
G. Heron
A. Malis
Tellabs
September 2006
Encapsulation Methods for Transport of
PPP/High-Level Data Link Control (HDLC) over MPLS Networks
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
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 a
Multiprotocol 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) within a pseudowire.
Table of Contents
1. Introduction ....................................................2
2. Specification of Requirements ...................................2
3. Applicability Statement .........................................5
4. General Encapsulation Method ....................................6
4.1. The Control Word ...........................................6
4.2. MTU Requirements ...........................................8
5. Protocol-Specific Details .......................................9
5.1. HDLC .......................................................9
5.2. Frame Relay Port Mode ......................................9
5.3. PPP .......................................................10
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 .....................................11
7. Congestion Control .............................................12
8. IANA Considerations ............................................12
9. Security Considerations ........................................12
10. Normative References ..........................................13
11. Informative References ........................................13
1. 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 PW has been set up by some means outside the
scope of this document. This may be via manual configuration, or
using a signaling protocol such as that defined in [RFC4447].
The following figure describes the reference models that are derived
from [RFC3985] to support the HDLC/PPP PW emulated services. The
reader is also assumed to be familiar with the content of the
[RFC3985] document.
2. 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 [RFC2119].
|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- 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 across the network, decapsulated at PE2, and transmitted
out on an attachment circuit at PE2.
The following reference model describes the termination point of each
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, and
policing. 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.
3. Applicability Statement
PPP/HDLC transport over PW service is not intended to emulate the
traditional PPP or HDLC service perfectly, but it can be used for
some applications that require PPP or HDLC transport service.
The applicability statements in [RFC4619] 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 frame
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
User Network Interface (UNI) or Network Node Interface (NNI) traffic.
Since all packets are passed in a largely transparent manner over the
HDLC PW, any protocol that has HDLC-like framing may use the HDLC PW
mode, including PPP, Frame-Relay, and X.25. Exceptions include cases
where direct access to the HDLC interface is required, or modes that
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
[RFC4447], is not affected by PPP Maximum Receive Unit (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.
4. General Encapsulation Method
This section describes the general encapsulation format for PPP and
HDLC packets over MPLS pseudowires.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pseudowire 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 Pseudowire Header identifies a particular PPP/HDLC service on a
tunnel. In case the of MPLS, the Pseudowire 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 [RFC4385]. It provides the ability
to sequence individual packets on the PW and avoidance of equal-cost
multiple-path load-balancing (ECMP) [RFC2992] and enables Operations
and Management (OAM) mechanisms, including [VCCV].
[RFC4385] 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 because ECMP
implementations may examine the first nibble after the MPLS label
stack to determine whether the content of the labeled packet is IP.
Thus, if the PPP protocol number of a 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 from that of 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, and strict packet sequencing may not be
required. 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.
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 [RFC4447].
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|FRG| 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
[RFC4385].
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 for
transmitting and MUST be ignored upon receipt.
The next 2 bits are defined in [RFC4623].
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 in transit on 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.[RFC4385]
The sequence number space is a 16-bit, unsigned circular space. The
sequence number value 0 is used to indicate an unsequenced
packet.[RFC4385]
The procedures described in Section 4 of [RFC4385] 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 [RFC4623] MAY be used to fragment encapsulated packets
that exceed the PSN MTU. However, if [RFC4623] 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 [RFC4447], it MUST be dropped. It is also
RECOMMENDED that PPP devices be configured to not negotiate PPP MRUs
larger than that of the AC MTU.
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 and control fields, but excluding HDLC flags and the
FCS. Bit/Byte stuffing is undone. If the OPTIONAL control word is
used, then the flag bits in the control word are not used and MUST be
set to 0 for 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 if 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 [RFC4447].
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 [RFC4446] 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 Virtual Circuits (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
Frame Relay (FR) device and a PE is either an FR UNI or an NNI. All
FR VCs carried over the interface are mapped into one HDLC PW. The
standard frame relay Link Management Interface (LMI) procedures
happen directly between the CEs. Thus with port mode, we have many-
to-one mapping between FR VCs and a PW.
+------+ +-------+
| 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. Pseudowires 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 [RFC4446]: 0x000F Frame-
Relay Port mode.
All other aspects of this PW type are identical to the HDLC PW
encapsulation described above.
5.3. PPP
PPP mode provides point-to-point transport of PPP-encapsulated
traffic, as specified in [RFC1661]. 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 for 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 if
the AC is administratively disabled, the PE MUST send the appropriate
PW status notification message that corresponds to the PPP AC status.
Note 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 [RFC4447].
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 [RFC4446] 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 [RFC3032] is simply prepended to the emulated PW encapsulation
and thus appears 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
[RFC4447]) 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.
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 [RFC3985], the PSN carrying the PW may be subject to
congestion, the characteristics of which are dependent upon 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 [RFC4447], then procedures specified in [RFC4447]
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 new IANA Actions. All necessary IANA actions
have already been included in [RFC4446].
9. Security Considerations
The PPP and HDLC pseudowire type is subject to all the general
security considerations discussed in [RFC3985][RFC4447]. 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. Normative References
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson,
"Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
for Use over an MPLS PSN", RFC 4385, February 2006.
[RFC4446] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC4619] Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
"Encapsulation Methods for Transport of Frame Relay over
Multiprotocol Label Switching (MPLS) Networks", RFC
4619, September 2006.
[RFC4623] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-
to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
August 2006.
11. Informative References
[Q922] ITU-T Recommendation Q.922 Specification for Frame Mode
Basic call control, ITU Geneva 1995.
[Q933] ITU-T Recommendation Q.933 Specification for Frame Mode
Basic call control, ITU Geneva 2003.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, November 2000.
[RFC3985] Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[VCCV] Nadeau, T., et al., "Pseudo Wire Virtual Circuit
Connection Verification (VCCV)", Work in Progress,
October 2005.
Contributing Author Information
Yeongil Seo
463-1 KT Technology Lab
Jeonmin-dong Yusung-gu
Daegeon, Korea
EMail: syi1@kt.co.kr
Toby Smith
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
EMail: tob@laurelnetworks.com
Authors' Addresses
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
EMail: lmartini@cisco.com
Giles Heron
Tellabs
Abbey Place
24-28 Easton Street
High Wycombe
Bucks
HP11 1NT
UK
EMail: giles.heron@tellabs.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
EMail: erosen@cisco.com
Andrew G. Malis
Tellabs
1415 West Diehl Road
Naperville, IL 60563
EMail: Andy.Malis@tellabs.com
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
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
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.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
ISO Plain Language
On this page
§1Carrying PPP and HDLC Across an MPLS Network
A PPP/HDLC pseudowire allows PPP and HDLC protocol data units, the individual frames of those link protocols, to be carried over an MPLS network without terminating the PPP or HDLC protocol. The document assumes the pseudowire has been set up by some means outside its scope, either by manual configuration or with a signalling protocol such as the one in RFC 4447.
The reference model has a customer edge device at each end, connected to a provider edge router, with a packet-switched network between the two provider edge routers. PE1 is the ingress router and PE2 is the egress router. A layer 2 packet is received at PE1, encapsulated there, transported across the network, decapsulated at PE2, and transmitted out on an attachment circuit at PE2.
The pseudowire terminates at a logical port within the provider edge router. That port provides a native service processing function which delivers each PPP or HDLC packet it receives, unaltered, to the corresponding point in the provider edge router at the other end of the pseudowire. The native service processing function may include bit stuffing, bridging between pseudowires, layer 2 encapsulation, shaping and policing. These functions are specific to the native packet technology and may not be needed for the emulated service.
Each end of the link has a customer device and a provider router. The provider router wraps the link's frames in MPLS labels, sends them across the packet-switched core, and takes the labels off again at the far end. The frame itself passes through unchanged, and neither end router runs the PPP or HDLC protocol.
- LAYER 2 PACKETReceived on the attachment circuit.
- MPLS LABEL, CONTROL WORD, PAYLOADThe frame is encapsulated and sent across the core.
- MPLS LABEL, CONTROL WORD, PAYLOAD
- LAYER 2 PACKETDecapsulated and sent out on the attachment circuit.
§3What the Emulation Does and Does Not Do
PPP/HDLC transport over a pseudowire is not intended to emulate the traditional PPP or HDLC service perfectly, but it can be used for some applications that require PPP or HDLC transport service.
The differences from a real link are these. 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, but that is outside the scope of this document. A Frame Relay port mode pseudowire, or an HDLC pseudowire, does not process any Frame Relay status messages or alarms. The HDLC flags are processed locally in the provider edge router connected to the attachment circuit.
The HDLC mode is suitable for port-to-port transport of Frame Relay user network interface or network node interface traffic. Since all packets are passed in a largely transparent manner over the HDLC pseudowire, any protocol that has HDLC-like framing may use it, including PPP, Frame Relay and X.25. Exceptions are cases where direct access to the HDLC interface is required, or modes that operate on the flags, the frame check sequence, or the bit and byte unstuffing that is performed before the HDLC packet is sent over the pseudowire. PPP asynchronous-control-character-map negotiation is an example of such a mode.
For PPP, because media-specific framing is not carried, three options will not operate correctly if the PPP peers attempt to negotiate them: frame check sequence alternatives, address-and-control-field compression, and asynchronous-control-character-map. PW LSP interface MTU negotiation is not affected by PPP maximum receive unit advertisement, so if a PPP peer sends a packet longer than the size negotiated for the pseudowire tunnel, that packet is discarded by the ingress router.
This is an emulation, not the real thing. Frame ordering only holds if the control word is used and carries a sequence number, and no implementation has to support that. Frame Relay alarms are ignored, and HDLC flags are handled by the router at each end rather than travelling. For PPP, three options cannot work: the frame check sequence alternatives, address-and-control-field compression, and the asynchronous control character map.
| Aspect | Traditional PPP or HDLC link | PPP/HDLC pseudowire |
|---|---|---|
| Packet ordering | Preserved by the link | Preserved only if the control word sequence field is used, and it is optional |
| Quality of service | Defined for the link | Can be emulated, but is outside the scope of this document |
| Frame Relay status messages THIS RFC | Processed | Not processed |
| HDLC flags THIS RFC | Present on the link | Processed locally in the provider edge router |
| PPP FCS alternatives, ACFC and ACCM THIS RFC | Negotiable | Do not operate correctly if negotiated |
§4The General Encapsulation Format
The general encapsulation format has four parts stacked on top of each other. The PSN transport header depends on the tunnelling technology in use and carries the encapsulated information through the packet-switched core. The pseudowire header identifies a particular PPP or HDLC service on a tunnel; in the case of MPLS it is the MPLS label at the bottom of the MPLS label stack. The control word is inserted before the PPP or HDLC service payload and may contain a length and a sequence number. Below that sits the PPP or HDLC service payload itself.
To use an MPLS label as the demultiplexer field, a 32-bit label stack entry is simply prepended to the emulated pseudowire encapsulation, so that it appears as the bottom label of an MPLS label stack. This label may be called the PW label. The particular emulated pseudowire identified by a particular label value must be agreed by the ingress and egress label switching routers, either by signalling or by configuration.
If quality of service information is to be carried, it should be represented in the EXP field of the PW label, and if the ingress router imposes more than one MPLS label, the EXP field of any labels higher in the stack must carry the same value. The ingress router must set the S bit of the PW label to 1, to denote that the PW label is at the bottom of the stack.
The encapsulated packet has up to four layers: whatever the tunnelling technology needs, the MPLS label that says which pseudowire this is, an optional control word, and the original link frame. The label goes at the bottom of the label stack and its bottom-of-stack bit is set to one.
§4.1The Control Word
Four requirements may need to be satisfied when transporting layer 2 protocols over an MPLS network: sequentiality may need to be preserved, small packets may need to be padded so they can travel on a medium whose minimum transport unit is larger than the packet, control bits carried in the header of the layer 2 packet may need to be transported, and an in-band associated channel may be needed for operation and maintenance communications.
The control word is based on the generic pseudowire MPLS control word defined in RFC 4385. It provides the ability to sequence individual packets on the pseudowire and to avoid equal-cost multiple-path load-balancing, and it enables operation and management mechanisms. That last point matters because equal-cost multiple-path implementations may examine the first nibble after the MPLS label stack to decide whether the content is IP. If the PPP protocol number of a packet carried without a control word begins with 0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 packet, which may lead to the packets of one pseudowire not following the same path.
The control word is 32 bits long. The first four bits are set to zero to indicate a control word. The next four bits carry protocol-specific flags, which are not used for HDLC or PPP and must be set to zero when transmitting and ignored on receipt. The next two bits are defined in RFC 4623. The next six bits are a length field: if the packet length, defined as the layer 2 payload plus the control word, is less than 64 bytes, the field must be set to the packet length, and otherwise it must be set to zero. A non-zero value is used to remove any padding added by the MPLS network, and that padding must be removed at the egress router before the packet is forwarded.
The last 16 bits are a sequence number that can be used to guarantee ordered packet delivery, although processing it is optional. The sequence number space is a 16-bit unsigned circular space, and the value 0 indicates an unsequenced packet.
The control word is four bytes in front of the payload. Its first bytes are fixed flags, then a six-bit length field that tells the far end how much padding the network added, and a two-byte sequence number that can be used to put the packets back in order. The control word itself is optional, but both ends must know whether the other one sends it.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
CW-INDICATOR | CW bits | 4 bits | 0 | Set to zero to indicate a control word. |
FLAGS | Protocol flags | 4 bits | 0 | Not used for HDLC or PPP; set to zero when transmitting and ignored on receipt. |
FRG | FRG | 2 bits | Two bits defined in RFC 4623. | |
LENGTH | Length | 6 bits | Packet length when below 64 bytes, otherwise zero; used to strip padding added in transit. | |
SEQUENCE-NUMBER | Sequence Number | 2 bytes | Optional sequence number in a 16-bit unsigned circular space; 0 means unsequenced. |
§4.2MTU Requirements
The network must be configured with an MTU that is sufficient to transport the largest encapsulation packets. When MPLS is used as the tunnelling protocol, for example, this is likely to be 12 or more bytes greater than the largest packet size. The methodology described in RFC 4623 may be used to fragment encapsulated packets that exceed the PSN MTU. However, if that document is not used, then if the ingress router determines that an encapsulated layer 2 packet exceeds the MTU of the tunnel through which it must be sent, the packet must be dropped.
If a packet is received on the attachment circuit that exceeds the interface MTU value signalled for the pseudowire, it must be dropped. It is also recommended that PPP devices be configured not to negotiate PPP maximum receive units larger than the MTU of the attachment circuit.
The network has to be able to carry the largest encapsulated packet, which is a little larger than the original frame. If a frame is still too big, it can be split using the method in RFC 4623, or it is dropped. A frame that is too big for the attachment circuit is always dropped.
§5Protocol-Specific Details
HDLC. The HDLC mode provides port-to-port transport of HDLC-encapsulated traffic. The HDLC packet is transported in its entirety, including the HDLC address and control fields, but excluding the HDLC flags and the frame check sequence, and the bit and byte stuffing is undone. If the optional control word is used, the flag bits in the control word are not used and must be set to zero when transmitting and ignored on receipt. When the provider edge router detects a status change on the attachment circuit, such as a physical link failure or an administratively disabled circuit, it must send the appropriate pseudowire status notification message. The pseudowire of type 0x0006 carries HDLC packets.
Frame Relay port mode. This is an optional capability, also known as many-to-one mapping. Between two ports of Frame Relay devices, a number of Frame Relay virtual circuits are configured. Replacing that physical interface with a pair of provider edge routers and a pseudowire between them maps all of the virtual circuits carried over the interface into one HDLC pseudowire, while the standard Frame Relay link management interface procedures still happen directly between the customer edge devices. The virtual circuits are not visible individually to a provider edge router, which processes the set assigned to a port as an aggregate. This mode shares the HDLC encapsulation but uses a different pseudowire type, 0x000F for Frame Relay port mode; all other aspects are identical to the HDLC pseudowire.
PPP. The PPP mode provides point-to-point transport of PPP-encapsulated traffic. The PPP packet is transported in its entirety, including the protocol field whether or not it is compressed using protocol field compression, but excluding any media-specific framing information such as the HDLC address and control fields or the frame check sequence. PPP negotiation status is transparent to the pseudowire and must not be communicated to the remote provider edge router. The pseudowire of type 0x0007 carries PPP packets.
HDLC mode carries the whole frame apart from the flags and the frame check sequence, and it undoes the bit stuffing before sending. PPP mode also drops the HDLC address and control fields. Frame Relay port mode bundles all the virtual circuits of one port into a single pseudowire, and the routers treat them as one lump rather than looking at each circuit.
- 0x0006HDLC — Carries HDLC packets, with the flags and the frame check sequence removed.
- 0x0007PPP — Carries PPP packets, with media-specific framing removed.
- 0x000FFrame Relay Port mode — Same encapsulation as HDLC mode, for many-to-one mapping of Frame Relay virtual circuits.
§7Congestion Control
The packet-switched network carrying the pseudowire may be subject to congestion, and during congestion it may exhibit packet loss that impacts the service carried by the pseudowire. Because PPP/HDLC pseudowires carry an unspecified type of service across the network, they cannot behave in the TCP-friendly manner prescribed by RFC 2914. In the presence of services that reduce their transmission rate, PPP/HDLC pseudowires will thus consume more than their fair share and should be halted.
Whenever possible, PPP/HDLC pseudowires should be run over traffic-engineered networks that provide bandwidth allocation and admission control. IntServ-enabled domains providing the Guaranteed Service, or DiffServ-enabled domains using expedited forwarding, are examples of such networks. The provider edge routers should monitor for congestion, using explicit congestion notification or by measuring packet loss, so that the service using the pseudowire may be maintained. When significant congestion is detected, the pseudowire should be administratively disabled. If the pseudowire was set up using the protocol in RFC 4447, the status notification procedures in that document can be used to stop packet transmission on the ingress router from the egress router. The pseudowire may then be restarted by manual intervention, or by automatic means after an appropriate waiting time.
When the core network is busy it drops packets, and this kind of traffic has no way to slow itself down politely, so it should be turned off. The provider edge routers are told to watch for congestion and disable the pseudowire when it is bad enough, and to run the pseudowire over a network that has reserved bandwidth where possible.
§9Security Considerations
The PPP and HDLC pseudowire type is subject to all the general security considerations discussed in RFC 3985 and RFC 4447. 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 here.
This document only defines how the frames are wrapped. Whether the traffic is protected depends on the protocol used to carry it across the network, and each protocol has its own security rules.