INTAREA J. Zhu
Internet Draft Intel
Intended status: Standards Track
Expires: July 4,2019
January 4, 2019
Generic Multi-Access (GMA) Convergence Encapsulation Protocols
draft-zhu-intarea-gma-00
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Abstract
Today, a device can be simultaneously connected to multiple
communication networks based on different technology implementations
and network architectures like WiFi, LTE, 5G, and DSL. In such
scenario, it is desirable to combine multiple access networks to
improve quality of experience. For example, an IP flow may be
delivered over both LTE and Wi-Fi network for higher end-to-end
throughput. In another example, lost packets may be retransmitted
when the device switches from Wi-Fi to LTE. However, such multi-
access optimization requires inserting additional control
information, e.g. Sequence Number, into each IP packet.
Unfortunately, there is no existing IP protocol for such purpose. As
a result, IP-over-IP tunneling has been used in today's solutions,
e.g. [LWIPEP] [GRE], to insert the GRE header, and then encode
additional control information in the GRE header fields, e.g. Key,
Sequence Number. However, there are two main drawbacks with this
approach: 1) IP-over-IP tunneling leads to higher overhead
especially for small packet. For example, the overhead of IP-over-
IP/GRE tunneling with both Key and Sequence Number is 32 Bytes,
which is 80% of a 40 Bytes TCP ACK packet; 2) It is difficult to
introduce new control fields using the existing GRE header format.
This document presents a new light-weight and flexible encapsulation
protocol to support multi-access solutions [ATSSS] [MAMS] [LWIPEP]
without IP-over-IP tunneling.
Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................3
3. GMA Trailer Format.............................................4
4. Fragmentation..................................................6
5. Concatenation..................................................7
6. Security Considerations........................................8
7. IANA Considerations............................................8
8. References.....................................................8
8.1. Normative References......................................8
8.2. Informative References....................................8
1. Introduction
Figure 1 shows the user-plane Generic Multi-Access (GMA) protocol
stack, which has been used in today's multi-access solutions [ATSSS]
[MAMS] [LWIPEP] [GRE].
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+-----------------------------------------------------+
| IP PDU |
|-----------------------------------------------------|
+--|-----------------------------------------------------|--+
| |-----------------------------------------------------| |
| | Convergence Sublayer | |
| |-----------------------------------------------------| |
| |-----------------------------------------------------| |
| | Adaptation | Adaptation | Adaptation | |
| | Sublayer | Sublayer | Sublayer | |
| | (optional) | (optional) | (optional) | |
| |-----------------------------------------------------| |
| | Access #1 IP | Access #2 IP | Access #3 IP | |
| +-----------------------------------------------------+ |
+-----------------------------------------------------------+
Figure 1: GMA User-Plane Protocol Stack
It consists of the following two Sublayers:
o Convergence sublayer: This layer performs multi-access specific
tasks, e.g., multi-link (path) aggregation, splitting/reordering,
lossless switching/retransmission, fragmentation, concatenation, etc.
o Adaptation sublayer: This layer performs functions to handle
tunneling, network layer security, and NAT.
The convergence sublayer operates on top of the adaptation sublayer in
the protocol stack. From the Transmitter perspective, a User Payload
(e.g. IP PDU) is processed by the convergence sublayer first, and then
by the adaptation sublayer before being transported over a delivery
connection; from the Receiver perspective, an IP packet received over a
delivery connection is processed by the adaptation sublayer first, and
then by the convergence sublayer.
This document presents a light-weight encapsulation protocol for the
convergence sublayer. It avoids IP-over-IP tunneling to minimize the
overhead, and introduces new control fields to support fragmentation
and concatenation at the convergence sublayer, which are not available
in today's GRE-based solutions [LWIPEP] [GRE].
2. Conventions used in this document
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].
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3. GMA Trailer Format
+------------------------------------------------------+
| IP hdr | IP payload | GMA Trailer |
+------------------------------------------------------+
Figure 2: Trailer-based Encapsulation PDU Format
Figure 2 shows the trailer-based encapsulation GMA PDU (protocol data
unit) format for the convergence sublayer. A (GMA) PDU MAY carry one or
multiple IP packets, aka (GMA) SDUs (service data unit), in the
payload, or a fragment of the SDU.
The GMA (Generic Multi-Access) trailer SHOULD consist of the following
two mandatory fields:
o Next Header (1 Byte): the IP protocol type of the (first) SDU in a
PDU
o Flags (1 Byte): Bit 0 is the least significant bit (LSB), and Bit 7
is the most significant bit (MSB).
+ Concatenation Present (bit 0): If the Concatenation Present bit
is set to 1, then the PDU carries multiple SDUs, and the First
SDU Length field is present and contains valid information.
+ Fragmentation Present (bit 1): If the Fragmentation Present bit
is set to 1, then the PDU carry a fragment of the SDU and the
Fragmentation Control field is present and contains valid
information.
+ Flow ID Present (bit 2): If the Flow ID Present bit is set to 1,
then the Flow ID field is present and contains valid
information.
+ Checksum Present (bit 3): If the Checksum Present bit is set to
1, then the Checksum field is present and contains valid
information.
+ Sequence Number Present (bit 4): If the Sequence Number Present
bit is set to 1, then the Sequence Number field is present and
contains valid information.
+ Timestamp Present (bit 5): If the Timestamp Present bit is set
to 1, then the Timestamp field is present and contains valid
information.
+ Bit 6-7: reserved
The GMA (Generic Multi-Access) trailer MAY consist of the following
optional fields:
o First SDU Length (2 Bytes): the length of the first IP packet in the
PDU, only included if a PDU contains multiple IP packets.
o Fragmentation Control (FC) (e.g. 1 Byte): to provide necessary
information for re-assembly, only needed if a PDU carries fragments.
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o Flow ID (1 Byte): an unsigned integer to identify the IP flow that a
PDU belongs to. It is composted of the following two sub-fields:
+ Connection ID (MSB 4 Bits): an unsigned integer to identify the
connection of the IP flow
+ Traffic Class ID (LSB 4 Bits): an unsigned integer to identify
the traffic class of the IP flow, for example Data Radio Bearer
(DRB) ID [LWIPEP] for a cellular (e.g. LTE) connection
o Checksum (1 Byte): to contain the (one's complement) checksum sum of
all the 8 bit in the trailer. For purposes of computing the checksum,
the value of the checksum field is zero. This field is present only
if the Checksum Present bit is set to one.
o Sequence Number (3 Bytes): an auto-incremented integer to indicate
order of transmission of the SDU (e.g. IP packet), needed for
lossless switching or multi-link (path) aggregation or fragmentation.
Sequence Number SHALL be generated per flow.
o Timestamp (4 Bytes): to contain the current value of the timestamp
clock of the transmitter in the unit of milliseconds.
Figure 3 shows the GMA trailer format with all the fields present. The
Next Header field is always the last octet at the end of the PDU, and
the Flags field is the second last. The Receiver SHOULD first decode
the Flags field to determine the length of the trailer, and then decode
each field accordingly.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow ID | FC | First SDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: GMA Trailer Format
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Moreover, the Protocol Type field in the IP header of the PDU SHOULD be
changed to "xyz" to indicate the presence of GMA trailer. The following
two IP header fields MAY also be changed:
o IP Length: add the length of "GMA Trailer" to the length of the
original IP packet
o IP checksum: recalculate after changing "Protocol Type" and "IP
Length"
4. Fragmentation
The convergence sublayer MAY support fragmentation if a delivery
connection has a smaller maximum transmission unit (MTU) than the
original IP packet (SDU). The fragmentation procedure at the
convergence sublayer is similar to IP fragmentation [RFC791] in
principle, but with the following two differences for less overhead:
o The fragment offset field is expressed in number of fragments not 8-
bytes blocks
o The maximum number of fragments per SDU is 2^7 (=128)
The Fragmentation Control (FC) field in the trailer contains the
following bits:
o Bit #7: a More Fragment (MF) flag to indicate if the fragment is the
last one (0) or not (1)
o Bit #0~#6: Fragment Offset (in units of fragments) to specify the
offset of a particular fragment relative to the beginning of the SDU
A PDU carries a whole SDU without fragmentation if the FC field is set
to all "0"s or the FC field is not present in the trailer. Otherwise,
the PDU contains a fragment of the SDU.
The Sequence Number (SN) field in the trailer is used to distinguish
the fragments of one SDU from those of another. The Fragment Offset
(FO) field tells the receiver the position of a fragment in the
original SDU. The More Fragment (MF) flag indicates the last fragment.
To fragment a long SDU, the transmitter creates two PDUs and copies the
content of the IP header fields from the long PDU into the IP header of
both PDUs. The length field in the IP header of PDU SHOULD be changed
to the length of the PDU, and the protocol type SHOULD be changed to
"xyz".
The data of the long SDU is divided into two portions based on the MTU
size of the delivery connection. The first portion of the data is
placed in the first PDU. The MF flag is set to "1", and the FO field is
set to "0". The second portion of the data is placed in the second PDU.
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The MF flag is set to "0", and the FO field is set to "1". This
procedure can be generalized for an n-way split, rather than the two-
way split described the above.
To assemble the fragments of a SDU, the receiver combines PDUs that all
have the same Sequence Number (in the trailer). The combination is done
by placing the data portion of each fragment in the relative order
indicated by the Fragment Offset in that fragment's trailer. The first
fragment will have the Fragment Offset set to "0", and the last
fragment will have the More-Fragments flag reset to "0".
5. Concatenation
The convergence sublayer MAY support concatenation if a delivery
connection has a larger maximum transmission unit (MTU) than the
original IP packet (SDU). Only the SDUs with the same client IP
address, and the same Flow ID MAY be concatenated.
The First SDU Length (FSL) field SHOULD be included in the trailer to
indicate the length of the first SDU.
To concatenate two or more SDUs, the transmitter creates one PDU and
copies the content of the IP header field from the first SDU into the
IP header of the PDU. The data of the first SDU is placed in the first
portion of the data of the PDU. The whole second SDU is then placed in
the second portion of the data of the PDU (Figure 4). The procedure
continues till the PDU size reaches the MTU of the delivery connection.
If the FSL field is present in the trailer, the IP length field of the
PDU SHOULD be updated to include all concatenated SDUs and the trailer,
and the IP checksum field SHOULD be recalculated.
To disaggregate a PDU, the receiver first obtains the length of the
first SDU from the FSL field in the trailer, and decodes the first SDU.
If the FSL field or the trailer is not present, the receiver obtains
the length of the first SDU directly from the IP length field of the
PDU. The receiver then obtains the length of the second SDU based on
the length field in the second SDU IP header, and decodes the second
SDU. The procedure continues till no byte is left in the PDU.
If a PDU contains multiple SDUs, the SN field in the trailer is for the
last SDU, and the SN of other SDU carried by the same PDU can be
obtained according to its order in the PDU. For example, if the SN
field is 6 and a PDU contains 3 SDUs (IP packets), the SN is 4, 5, and
6 for the first, second, and last SDU respectively.
+------------------------------------------------------------+
| IP hdr | IP payload | IP hdr | IP payload | GMA Trailer |
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+------------------------------------------------------------+
Figure 4: GMA PDU Format with Concatenation
6. Security Considerations
The convergence sublayer relies on the security of the underlying
network transport paths. When this cannot be assumed, appropriate
security protocols (IPsec, DTLS, etc.) SHOULD be configured at the
adaptation sublayer.
7. IANA Considerations
The trailer based encapsulation protocol in this document requires the
allocation of a new value in the "IP Protocol Type" field within the IP
header (e.g. xyz GMA Generic-Multi-Access-convergence-encapsulation).
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[MAMS] S. Kanugovi, S. Vasudevan, F. Baboescu, and J. Zhu, "Multiple
Access Management Protocol",
https://tools.ietf.org/html/draft-kanugovi-intarea-mams-
protocol-03
[IANA] https://www.iana.org/assignments/protocol-
numbers/protocol-numbers.xhtml
[LWIPEP] 3GPP TS 36.361, "Evolved Universal Terrestrial Radio Access
(E-UTRA); LTE-WLAN Radio Level Integration Using Ipsec
Tunnel (LWIP) encapsulation; Protocol specification"
[RFC791] Internet Protocol, September 1981
[ATSSS] 3GPP TR 23.793, Study on access traffic steering, switch and
splitting support in the 5G system architecture.
[GRE] RFC 8157, Huawei's GRE Tunnel Bonding Protocol, May 2017
Authors' Addresses
Jing Zhu
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Intel
Email: jing.z.zhu@intel.com
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