Ipsilon IPv4 over ATM: Transmission of Flow Labelled IPv4 on ATM Data Links Ipsilon Version 1.0
How to send labeled IPv4 packets over ATM links.
Newman, P., Edwards, W., Hinden, R., Hoffman, E., Ching Liaw, F., Lyon, T., and G. Minshall, "Transmission of Flow Labelled IPv4 on ATM Data Links Ipsilon Version 1.0", RFC 1954, DOI 10.17487/RFC1954, May 1996, <https://www.rfc-editor.org/info/rfc1954>.
@misc{rfc1954,
series = {Request for Comments},
number = 1954,
howpublished = {RFC 1954},
publisher = {RFC Editor},
doi = {10.17487/RFC1954},
url = {https://www.rfc-editor.org/info/rfc1954},
author = {P. Newman and W. Edwards and R. Hinden and E. Hoffman and F. Ching Liaw and T. Lyon and G. Minshall},
title = {{Transmission of Flow Labelled IPv4 on ATM Data Links Ipsilon Version 1.0}},
pagetotal = 8,
year = 1996,
month = may,
} RFC text
Contents
Network Working Group P. Newman, Ipsilon
Request for Comments: 1954 W. L. Edwards, Sprint
Category: Informational R. Hinden, Ipsilon
E. Hoffman, Ipsilon
F. Ching Liaw, Ipsilon
T. Lyon, Ipsilon
G. Minshall, Ipsilon
May 1996
Transmission of Flow Labelled IPv4 on ATM Data Links
Ipsilon Version 1.0
Status of this Memo
This document provides information for the Internet community. This
memo does not specify an Internet standard of any kind. Distribution
of this memo is unlimited.
IESG Note:
This note documents a private protocol for IPv4-based flows. This
protocol is NOT the product of an IETF working group nor is it a
standards track document. It has not necessarily benefited from the
widespread and in depth community review that standards track
documents receive.
Abstract
This document specifies the manner for transmitting IPv4 datagrams
over an ATM data link, both in a default manner and in the presence
of flow labelling via Ipsilon Flow Management Protocol [IFMP].
Table of Contents
Introduction....................................................2
1. Labels.......................................................2
2. Default Encapsulation........................................2
3. Flow Type 0 Encapsulation....................................3
4. Flow Type 1 Encapsulation....................................4
5. Flow Type 2 Encapsulation....................................5
References......................................................7
Security Considerations.........................................7
Authors' Addresses..............................................7
Introduction
This document specifies the manner for transmitting IPv4 datagrams
over an ATM data link, both in a default manner and in the presence
of flow labelling via Ipsilon Flow Management Protocol [IFMP]. ATM
specific functions such as OAM cells, the CLP bit, and ABR RM cells
are not used. There are no reserved VCIs other than VPI = 0, VCI =
0, which indicates an unassigned cell; and VPI = 0, VCI = 15, which
is used for the default encapsulation. IFMP messages must be sent
using the default encapsulation.
1. Labels
Labels, as carried by IFMP, are realized on an ATM data link as
specific VPI/VCIs. The format of the Label field for ATM labels is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Reservd| VPI | VCI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The low order 16 bits of the label correspond to the VCI, with the
least significant bit of the Label field corresponding to the least
significant bit of the VCI. If the link cannot support a full 16 bit
VCI the unused bits of the VCI must be the most significant bits and
they must be set to zero.
The next 12 higher order bits of the label correspond to the VPI,
with the least significant of these bits corresponding to the least
significant bit of the VPI. If the link cannot support a full 12 bit
VPI, then the unused bits of the VPI must be the most significant
bits and they must be set to zero. The most significant four bits of
the label are reserved. They should be set to zero by the sender and
ignored by the receiver.
2. Default Encapsulation
The default encapsulation for IPv4 packets on ATM data links is the
LLC/SNAP encapsulation specified in section 4.1 "LLC encapsulation
for routed protocols" of RFC 1483 [RFC1483]. Such frames begin with
the octets 0xAA 0xAA 0x03 0x00 0x00 0x00 0x08 0x00 (the LLC/SNAP
header for IPv4). The LLC/SNAP header is prefixed to the IP datagram
and the entire packet is encapsulated within the payload of an AAL-5
CPCS-PDU as specified in RFC 1483 and illustrated below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LLC (0xAA-AA-03) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| SNAP (0x00-00-00-08-00) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IPv4 Datagram ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad (0 - 47 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AAL-5 CPCS-PDU Trailer (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The maximum transmission unit (MTU) of an IPv4 datagram using the
default encapsulation is 1500 octets.
Frames using the default encapsulation are sent to: VPI = 0, VCI =
15.
3. Flow Type 0 Encapsulation
All IPv4 frames using Flow Type 0 are encapsulated within the payload
of an AAL-5 CPCS-PDU. This is the null encapsulation of section 5.1
"VC based multiplexing of routed protocols" from RFC 1483 [RFC1483].
There is no LLC/SNAP header. The first octet of the frame
corresponds to the first octet of the IPv4 datagram (i.e., the octet
that contains the IP version number (4) and Internet Header Length
(IHL) ). The IP datagram is encapsulated within the payload of an
AAL-5 CPCS-PDU as specified in RFC 1483 and illustrated below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IPv4 Datagram ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad (0 - 47 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AAL-5 CPCS-PDU Trailer (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MTU of an IPv4 datagram using Flow Type 0 encapsulation is 1500
octets.
Frames using Flow Type 0 encapsulation are sent to the VPI/VCI
specified in the Label field of the corresponding IFMP Flow Type 0
Redirect message element [IFMP].
4. Flow Type 1 Encapsulation
All IPv4 frames using Flow Type 1 are encapsulated directly in the
payload of an AAL-5 CPCS-PDU. This is the null encapsulation of
section 5.1 "VC based multiplexing of routed protocols" from RFC 1483
[RFC1483]. There is no LLC/SNAP header. Also, the following fields
of the IP header are not transmitted: Version, Internet Header Length
(IHL), Type of Service (TOS), Time to Live (TTL), Protocol, Source
Address, and Destination Address. In addition, the first 4 octets
immediately following the IP header (as determined by the IHL field)
are not transmitted. (These 4 octets correspond to the source and
destination ports for TCP and UDP datagrams.) The value of the Total
Length field is not changed; it remains the total length of the IP
datagram before the above fields were removed. The transmitted value
of the Checksum field is the checksum value that would have been
computed for the entire IP header if the TTL field had been set to
zero (i.e., the actual value of the TTL field is "subtracted", using
one's-complement arithmetic, from the Checksum before transmission).
The IP datagram is encapsulated within the payload of an AAL-5 CPCS-
PDU as specified in RFC 1483 and illustrated below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Flags| Fragment Offset | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad (0 - 47 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AAL-5 CPCS-PDU Trailer (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The MTU of the IPv4 datagram using Flow Type 1 encapsulation is 1484
octets (1500 octets minus the 16 octets specified above).
Frames using Flow Type 1 encapsulation are sent to the VPI/VCI
specified in the Label field of the corresponding IFMP Flow Type 1
redirect message element [IFMP].
5. Flow Type 2 Encapsulation
All IPv4 frames using Flow Type 2 are encapsulated directly in the
payload of an AAL-5 CPCS-PDU. This is the null encapsulation of
section 5.1 "VC based multiplexing of routed protocols" from RFC 1483
[RFC1483]. There is no LLC/SNAP header. Also, the following fields
of the IP header are not transmitted: Version, Internet Header Length
(IHL), Time to Live (TTL), Source Address, and Destination Address.
The first 4 octets immediately following the IP header (as determined
by the IHL field) are transmitted. (These 4 octets correspond to the
source and destination ports for TCP and UDP datagrams.) The value of
the Total Length field is not changed; it remains the total length of
the IP datagram before the above fields were removed. The
transmitted value of the Checksum field is the checksum value that
would have been computed for the entire IP header if the TTL field
had been set to zero (i.e., the actual value of the TTL field is
"subtracted", using one's-complement arithmetic, from the Checksum
before transmission).
The IP datagram is encapsulated within the payload of an AAL-5 CPCS-
PDU as specified in RFC 1483 and illustrated below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Protocol | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pad (0 - 47 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ AAL-5 CPCS-PDU Trailer (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Reserved fields are not used and should be set to zero by the
sender and ignored by the receiver.
The MTU of the IPv4 datagram using Flow Type 2 encapsulation is 1492
octets (1500 octets minus the 8 octets specified above).
Frames using Flow Type 2 encapsulation are sent to the VPI/VCI
specified in the Label field of the corresponding IFMP Flow Type 2
redirect message element [IFMP].
References
[IFMP] Newman, P., et. al., "Ipsilon Flow Management Protocol
Specification for IPv Version 1.0", Ipsilon Networks
Inc., RFC 1953, May 1996.
[RFC1483] Heinanen, J., "Multiprotocol Encapsulation over ATM
Adaptation Layer 5", RFC 1483, July 1993.
SECURITY CONSIDERATIONS
Security issues are not discussed in this document.
AUTHORS' ADDRESSES
Peter Newman Phone: +1 (415) 846-4603
Ipsilon Networks, Inc. EMail: pn@ipsilon.com
W. L. Edwards, Chief Scientist Phone: +1 (913) 534 5334
Sprint EMail: texas@sprintcorp.com
Robert M. Hinden Phone: +1 (415) 846-4604
Ipsilon Networks, Inc. EMail: hinden@ipsilon.com
Eric Hoffman Phone: +1 (415) 846-4610
Ipsilon Networks, Inc. EMail: hoffman@ipsilon.com
Fong Ching Liaw Phone: +1 (415) 846-4607
Ipsilon Networks, Inc. EMail: fong@ipsilon.com
Tom Lyon Phone: +1 (415) 846-4601
Ipsilon Networks, Inc. EMail: pugs@ipsilon.com
Greg Minshall Phone: +1 (415) 846-4605
Ipsilon Networks, Inc. EMail: minshall@ipsilon.com
Ipsilon Networks, Inc. is located at:
2191 East Bayshore Road
Suite 100
Palo Alto, CA 94303
USA
Sprint is located at:
Sprint
Sprint Technology Services - Long Distance Division
9300 Metcalf Avenue
Mailstop KSOPKB0802
Overland Park, KS 66212-6333
USA
AI ELI5
On this page
§1Introduction
This document specifies how to transmit IPv4 datagrams over an ATM data link in two scenarios: in the default case without labels, and when flow labels are applied via the Ipsilon Flow Management Protocol (IFMP, described in RFC 1953).
ATM is a connection-oriented network technology where data is sent in small fixed-size cells over virtual circuits identified by VPI (Virtual Path Identifier) and VCI (Virtual Circuit Identifier) pairs. The default encapsulation uses LLC/SNAP headers and is always available. When IFMP is used to label flows, optimized encapsulations can omit certain redundant IPv4 header fields to reduce overhead.
The document defines the label format for ATM and three flow-specific encapsulations corresponding to the three flow types defined in RFC 1953.
ATM is like a fast express postal system with many sorting bins. This RFC explains how to wrap internet packets so they fit properly in ATM's bins and how to handle shortcuts for regular mail runs.
§2Labels on ATM
In IFMP, a label is a 32-bit value. On ATM, labels are mapped to VPI/VCI pairs: the low 16 bits of the label correspond to the VCI, and the next 12 bits correspond to the VPI. The high 4 bits are reserved and must be set to zero.
If a link cannot support the full 12-bit VPI or 16-bit VCI, the unused bits must be the most significant bits and must be set to zero.
The label space is managed at each end of a link, and each label maps to a virtual circuit on that link.
A label is just a pair of numbers that pick which bin and which slot within that bin to use on the ATM network.
§3Default Encapsulation
The default encapsulation for IPv4 on ATM is the LLC/SNAP encapsulation specified in RFC 1483. Every IPv4 packet is prefixed with an 8-byte LLC/SNAP header (bytes 0xAA 0xAA 0x03 0x00 0x00 0x00 0x08 0x00) that identifies the protocol as IPv4.
The entire packet (header plus IPv4 datagram) is then encapsulated within the payload of an AAL-5 CPCS-PDU (ATM Adaptation Layer 5 Common Part Convergence Sublayer Protocol Data Unit). AAL-5 adds padding (0 to 47 octets) and an 8-octet trailer.
The maximum transmission unit (MTU) for IPv4 datagrams using the default encapsulation is 1500 octets. Default encapsulation frames are sent to VPI = 0, VCI = 15.
IFMP protocol messages must use the default encapsulation.
The default way is to add a label and an envelope to the internet packet, like putting a certified letter in a box. The biggest letter is 1500 bytes.
§4Flow Type 0: Changing Encapsulation
Flow Type 0 uses null encapsulation: the IPv4 datagram is placed directly in the AAL-5 payload with no LLC/SNAP header. This encapsulation is used when redirecting all IPv4 traffic, without regard to source or destination.
Because Flow Type 0 matches all IPv4 packets (regardless of source, destination, port, or other fields), it offers a simple way to apply a label to general IPv4 traffic.
The MTU is 1500 octets, and frames are sent to the VPI/VCI specified in the IFMP Flow Type 0 Redirect message.
Flow Type 0 is the simplest: just put the internet packet directly in the envelope, without the certified mail label, for very general traffic.
§5Flow Types 1 and 2: Optimized Encapsulations
Flow Type 1 uses null encapsulation and omits several redundant IPv4 header fields: Version, IHL, Type of Service, TTL, Protocol, and both addresses. It also omits the first 4 octets after the IP header (the source and destination ports for TCP and UDP). The MTU is reduced to 1484 octets (1500 minus 16 bytes).
Flow Type 2 is similar but keeps the source and destination ports and omits only Version, IHL, TTL, and the addresses. Its MTU is 1492 octets (1500 minus 8 bytes).
Both use the checksum value that would result if the TTL field were zero. This allows the receiver to verify the checksum even though the TTL may differ.
These optimized encapsulations reduce overhead for labeled flows by removing header information that is already known from the IFMP flow identifier (which includes addresses and ports for both Type 1 and Type 2).
For regular conversations, we can skip writing down the address and some other stuff because both routers already know it from the label. This saves space in the envelope.
| Encapsulation | LLC/SNAP Header | Omitted Fields | MTU |
|---|---|---|---|
| Default | Present (8 bytes) | None | 1500 octets |
| Flow Type 0 | None | None | 1500 octets |
| Flow Type 1 THIS RFC | None | Version, IHL, TOS, TTL, Protocol, addresses, ports | 1484 octets |
| Flow Type 2 THIS RFC | None | Version, IHL, TTL, addresses | 1492 octets |