Network Working Group Luca Martini
Internet Draft Nasser El-Aawar
Expiration Date: October 2002 Level 3 Communications, LLC.
Giles Heron Toby Smith
PacketExchange Ltd. Laurel Networks, Inc.
Alex Hamilton Andrew G. Malis
Daniel Tappan Vivace Networks, Inc.
Eric C. Rosen
Cisco Systems, Inc.
April 2002
Encapsulation Methods for Transport of PPP/HDLC Frames Over IP and MPLS Networks
draft-martini-ppp-hdlc-encap-mpls-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document describes methods for encapsulating the Protocol Data
Units (PDUs) of layer 2 protocols such as PPP, or HDLC for transport
across an MPLS or IP network.
Martini, et al. [Page 1]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
Table of Contents
1 Specification of Requirements .......................... 2
2 Introduction ........................................... 2
3 General encapsulation method ........................... 3
3.1 The Control Word ....................................... 3
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 ................................................... 6
4.2 PPP .................................................... 6
5 Using an MPLS Label as the Demultiplexer Field ......... 7
5.1 MPLS Shim EXP Bit Values ............................... 7
5.2 MPLS Shim S Bit Value .................................. 7
5.3 MPLS Shim TTL Values ................................... 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
In an MPLS or IP network, it is possible to use control protocols
such as those specified in [1] to set up "emulated virtual circuits"
that carry the the Protocol Data Units of layer 2 protocols across
the network. A number of these emulated virtual circuits may be
carried in a single tunnel. This requires of course that the layer 2
PDUs be encapsulated. We can distinguish three layers of this
encapsulation:
- the "tunnel header", which contains the information needed to
transport the PDU across the IP or MPLS network; this is header
belongs to the tunneling protocol, e.g., MPLS, GRE, L2TP.
Martini, et al. [Page 2]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
- the "demultiplexer field", which is used to distinguish
individual emulated virtual circuits within a single tunnel; this
field must be understood by the tunneling protocol as well; it
may be, e.g., an MPLS label or a GRE key field.
- the "emulated VC encapsulation", which contains the information
about the enclosed layer 2 PDU which is necessary in order to
properly emulate the corresponding layer 2 protocol.
This document specifies the emulated VC 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 VC encapsulation when an MPLS label is used
as the demultiplexer field.
QoS related issues are not discussed in this draft
For the purpose of this document R1 will be defined as the ingress
router, and R2 as the egress router. A layer 2 PDU will be received
at R1, encapsulated at R1, transported, decapsulated at R2, and
transmitted out of R2.
3. General encapsulation method
In most cases, it is not necessary to transport the layer 2
encapsulation across the network; rather, the layer 2 header can be
stripped at R1, and reproduced at R2. This is done using information
carried in the control word (see below), as well as information that
may already have been signaled from R1 to R2.
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.
Martini, et al. [Page 3]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
-iii. Control bits carried in the header of the layer 2 frame may
need to be transported.
The control word defined here addresses all three of these
requirements. For some protocols this word is REQUIRED, and for
others OPTIONAL. For protocols where the control word is OPTIONAL
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsvd | Flags |0 0| Length | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In the above diagram the first 4 bits are reserved for future use.
They MUST be set to 0 when transmitting, and MUST be ignored upon
receipt.
The next 4 bits provide space for carrying protocol specific flags.
These are defined in the protocol-specific details below.
The next 2 bits MUST be set to 0 when transmitting.
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
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.
Martini, et al. [Page 4]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
3.1.1. Setting the sequence number
For a given emulated VC, and a pair of routers R1 and R2, if R1
supports packet sequencing then the following procedures should be
used:
- the initial packet transmitted on the emulated VC MUST use
sequence number 1
- subsequent packets MUST increment the sequence number by one for
each packet
- when the transmit sequence number reaches the maximum 16 bit
value (65535) the sequence number MUST wrap to 1
If the transmitting router R1 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 R2 supports receive sequence number processing, then the
following procedures should be used:
When an emulated VC is initially set up, the "expected sequence
number" associated with it MUST be initialized to 1.
When a packet is received on that emulated VC, the sequence number
should be processed as follows:
- if the sequence number on the packet is 0, then the packet passes
the sequence number check
- otherwise if the packet sequence number >= the expected sequence
number and the packet sequence number - the expected sequence
number < 32768, then the packet is in order.
- otherwise if the packet sequence number < the expected sequence
number and the expected sequence number - the packet sequence
number >= 32768, then the packet is in order.
- otherwise the packet is out of order.
If a packet passes the sequence number check, or is in order then, it
can be delivered immediately. If the packet is in order, then the
expected sequence number should be set using the algorithm:
expected_sequence_number := packet_sequence_number + 1 mod 2**16
if (expected_sequence_number = 0) then expected_sequence_number := 1;
Martini, et al. [Page 5]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
Packets which are received out of order MAY be dropped or reordered
at the discretion of the receiver.
If a router R2 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
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:
Martini, et al. [Page 6]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
- Frame Check Sequence (FCS) Alternatives
- Address-and-Control-Field-Compression (ACFC)
- Asynchronous-Control-Character-Map (ACCM)
Note also that VC 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 VC LSP 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 VC encapsulation, and
hence will appear as the bottom label of an MPLS label stack. This
label may be called the "VC label". The particular emulated VC
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 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
VC 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, R1, MUST set the S bit of the VC label to a value of
1 to denote that the VC label is at the bottom of the stack.
Martini, et al. [Page 7]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
5.3. MPLS Shim TTL Values
The ingress LSR, R1, SHOULD set the TTL field of the VC label to a
value of 2.
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", draft-martini-
l2circuit-trans-mpls-09.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-martini-atm-encap-mpls-00.txt. ( work in
progress )
[5] "Encapsulation Methods for Transport of Ethernet Frames Over IP
and MPLS Networks", draft-martini-ethernet-encap-mpls-00.txt. ( work
in progress )
[6] "Encapsulation Methods for Transport of Frame-Relay Over IP and
MPLS Networks", draft-martini-frame-encap-mpls-00.txt. ( work in
progress )
Martini, et al. [Page 8]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
9. Author Information
Luca Martini
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: luca@level3.net
Nasser El-Aawar
Level 3 Communications, LLC.
1025 Eldorado Blvd.
Broomfield, CO, 80021
e-mail: nna@level3.net
Giles Heron
PacketExchange Ltd.
The Truman Brewery
91 Brick Lane
LONDON E1 6QL
United Kingdom
e-mail: giles@packetexchange.net
Dan Tappan
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: tappan@cisco.com
Alex Hamilton,
Cisco Systems Inc.
285 W. Tasman , MS-SJCI/3/4,
San Jose, CA, 95134
e-mail: tahamilt@cisco.com
Eric Rosen
Cisco Systems, Inc.
250 Apollo Drive
Chelmsford, MA, 01824
e-mail: erosen@cisco.com
Martini, et al. [Page 9]
Internet Draft draft-martini-ppp-hdlc-encap-mpls-00.txt April 2002
Toby Smith
Laurel Networks, Inc.
Omega Corporate Center
1300 Omega Drive
Pittsburgh, PA 15205
e-mail: tob@laurelnetworks.com
Andrew G. Malis
Vivace Networks, Inc.
2730 Orchard Parkway
San Jose, CA 95134
e-mail: Andy.Malis@vivacenetworks.com
Martini, et al. [Page 10]