Internet DRAFT - draft-zhu-intarea-gma
draft-zhu-intarea-gma
Network Working Group J. Zhu
Internet Draft Intel
Intended status: Experimental S. Kanugovi
Expires: May 24,2022 Nokia
November 24, 2021
Generic Multi-Access (GMA) Encapsulation Protocol
draft-zhu-intarea-gma-14
Abstract
A device can be simultaneously connected to multiple networks,
e.g., Wi-Fi, LTE, 5G, and DSL. It is desirable to seamlessly
combine the connectivity over these networks below the transport
layer (L4) to improve quality of experience for applications that
do not have built in multi-path capabilities. Such optimization
requires additional control information, e.g., a sequence number,
in each packet. This document presents a new light weight and
flexible encapsulation protocol for this need. The solution has
been developed by the authors based on their experiences in
multiple standards bodies including the IETF and 3GPP, is not an
Internet Standard and does not represent the consensus opinion of
the IETF. This document will enable other developers to build
interoperable implementations in order to experiment with the
protocol.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on May 24, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction ................................................. 2
1.1. Scope of Experiment ....................................4
2. Conventions used in this document ............................ 5
3. Use Case ..................................................... 5
4. GMA Encapsulation Methods .................................... 7
4.1. Trailer-based IP Encapsulation .........................8
4.2. Header-based IP Encapsulation .........................11
4.3. (Header-based) non-IP Encapsulation ...................11
4.4. IP Protocol Identifier ................................12
5. Fragmentation ............................................... 12
6. Concatenation ............................................... 14
7. Security Considerations ..................................... 15
8. IANA Considerations ......................................... 15
9. References .................................................. 16
9.1. Normative References ..................................16
9.2. Informative References ................................16
1. Introduction
A device can be simultaneously connected to multiple networks,
e.g., Wi-Fi, LTE, 5G, and DSL. It is desirable to seamlessly
combine the connectivity over these networks below the transport
layer (L4) to improve quality of experience for applications that
do not have built in multi-path capabilities.
Figure 1 shows the Multi-Access Management Service (MAMS) user-
plane protocol stack [MAMS], which has been used in today's multi-
access solutions [ATSSS] [LWIPEP] [GRE1] [GRE2]. It consists of
two layers: convergence and adaptation.
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The convergence layer is responsible for multi-access operations,
including multi-link (path) aggregation, splitting/reordering,
lossless switching/retransmission, fragmentation, concatenation,
etc. It operates on top of the adaptation layer in the protocol
stack. From the perspective of a transmitter, a User Payload
(e.g., IP packet) is processed by the convergence layer first, and
then by the adaptation layer before being transported over a
delivery connection; from the receiver's perspective, an IP packet
received over a delivery connection is processed by the adaptation
layer first, and then by the convergence layer.
+-----------------------------------------------------+
| User Payload, e.g., IP Protocol Data Unit (PDU) |
+-----------------------------------------------------+
+-----------------------------------------------------------+
| +-----------------------------------------------------+ |
| | Multi-Access (MX) Convergence Layer | |
| +-----------------------------------------------------+ |
| +-----------------------------------------------------+ |
| | MX Adaptation | MX Adaptation | MX Adaptation | |
| | Layer | Layer | Layer | |
| +-----------------+-----------------+-----------------+ |
| | Access #1 IP | Access #2 IP | Access #3 IP | |
| +-----------------------------------------------------+ |
| MAMS User-Plane Protocol Stack |
+-----------------------------------------------------------+
Figure 1: MAMS User-Plane Protocol Stack [MAMS]
GRE (Generic Routing Encapsulation) can be used [LWIPEP] [GRE1]
[GRE2] as the encapsulation protocol at the convergence layer to
encode additional control information, e.g., Key, Sequence Number.
However, there are two main drawbacks with this approach:
o It is difficult to introduce new control fields because the
GRE header formats are already defined,
o IP-over-IP tunnelling (required for GRE) leads to higher
overhead especially for small packet.
For example, the overhead of IP-over-IP/GRE tunnelling with both
Key and Sequence Number is 32 Bytes (20 Bytes IP header + 12 Bytes
GRE header), which is 80% of a 40 Bytes TCP ACK packet.
This document presents a light-weight GMA (Generic Multi-Access)
encapsulation protocol for the convergence layer. It supports
three encapsulation methods: trailer-based IP encapsulation,
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header-based IP encapsulation, and non-IP encapsulation.
Particularly, the IP encapsulation methods avoid IP-over-IP
tunneling overhead (20 Bytes), which is 50% of a 40 Bytes TCP ACK
packet. Moreover, it introduces new control fields to support
fragmentation and concatenation, which are not available in GRE-
based solutions [LWIPEP] [GRE1] [GRE2].
The GMA protocol only operates between endpoints that have been
configured to use GMA. This configuration can be through any
control messages and procedures, including, for example, Multi-
Access Management Services [MAMS]. Moreover, UDP or IPSec
tunneling can be used at the adaptation sublayer to protect GMA
operation from intermediate nodes.
The solution described in this document was been developed by the
authors based on their experiences in multiple standards bodies
including the IETF and 3GPP. However, it is not an Internet
Standard and does not represent the consensus opinion of the IETF.
This document presents the protocol specification to enable
experimentation as described in Section 1.1 and to facilitate
other interoperable implementations.
1.1. Scope of Experiment
The protocol described in this document is an experiment. The
objective of the experiment is to determine whether the protocol
meets the requirements, can be safely used, and has support for
deployment.
Section 4 describes three possible encapsulation methods that are
enabled by this protocol. Part of this experiment is to assess
whether all three mechanisms are necessary, or whether, for
example, all implementations are able to support the main
"trailer-based" IP encapsulation method. Similarly, the experiment
will investigate the relative merits of the IP and non-IP
encapsulation methods.
It is expected that this protocol experiment can be conducted on
the Internet since the GMA packets are identified by an IP
protocol number and the protocol is intended for single hop
propagation: devices should not be forwarding packet and if they
do they will not need to examine the payload, while destination
systems that do not support this protocol should not receive such
packets and will handle them as unknown payloads according to
normal IP processing. Thus, experimentation is conducted between
consenting end systems that have been mutually configured to
participate in the experiment as described in Section 7.
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Note that this experiment "re-uses" the IP protocol identifier 114
as described in Section 4.4. Part of this experiment is to assess
the safety of doing this. The experiment should consider the
following safety mechanisms:
o GMA endpoints SHOULD detect non-GMA IP packets that also use
114 and log an error to report the situation (although such
error logging MUST be subject to rate limits).
o GMA endpoints SHOULD stop using 114 and switch to non-IP
(UDP) based encapsulation (Sec 4.3, Figure 7) after detecting
any non-GMA usage of 114.
The experiment SHOULD use packet tracing tool, e.g., WireShark,
TCPDUMP, to monitor both ingress and egress traffic at GMA
endpoints and ensure the above safety mechanisms are implemented.
Path quality measurements (one-way-delay, loss, etc.) and
congestion detection are performed by receiver based on the GMA
control fields, e.g., sequence number, timestamp, etc. Receiver
will then dynamically control how to split or steer traffic over
multiple delivery connections accordingly. GMA control protocol
[GMAC] MAY be used for signaling between GMA endpoints. Another
objective of the experiment is to evaluate the usage of various
receiver-based algorithms [GCC] [MPIP] in multi-path traffic
management, and the impact on the e2e performance (throughput,
delay, etc.) of higher layer (transport) protocols, e.g., TCP,
QUIC, WebRTC, etc.
The authors will continually assess the progress of this
experiment and encourage other implementers to contact them to
report the status of their implementations and their experiences
with the protocol.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
3. Use Case
As shown in Figure 2, a client device (e.g., Smartphone, Laptop,
etc.) may connect to the Internet via both Wi-Fi and LTE
connections, one of which (e.g., LTE) may operate as the anchor
connection, and the other (e.g., Wi-Fi) may operate as the
delivery connection. The anchor connection provides the IP address
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and connectivity for end-to-end Internet access, and the delivery
connection provides an additional path between client and Multi-
Access Gateway for multi-access optimizations.
Multi-Access Aggregation
+---+ +---+
| |A|--- LTE Connection -----|C| |
|U|-| |-|S| Internet
| |B|--- Wi-Fi Connection ---|D| |
+---+ +---+
Client Multi-Access Gateway
A: The adaptation layer endpoint of the LTE connection
resides in the client
B: The adaptation layer endpoint of the Wi-Fi connection
resides in the client
C: The adaptation layer endpoint of the LTE connection
resides in the Multi-Access Gateway, aka N-MADP (Network
Multi-Access Data Proxy) in [MAMS]
D: The adaptation layer endpoint of the Wi-Fi connection
resides in the Multi-Access Gateway
U: The convergence layer endpoint resides in the client
S: The convergence layer endpoint resides in the Multi-
Access Gateway
Figure 2: GMA-based Multi-Access Aggregation
For example, per-packet aggregation allows a single IP flow to use
the combined bandwidth of the two connections. In another example,
packets lost due to a temporarily link outage may be
retransmitted. Moreover, packets may be duplicated over multiple
connections to achieve high reliability and low latency, where
duplicated packets are eliminated by the receiving side. Such
multi-access optimization requires additional control information,
e.g., a sequence number, in each packet, which can be supported by
the GMA encapsulation protocol described in this document.
The GMA protocol described in this document is designed for
multiple connections, but it may also be used when there is only
one connection between two endpoints. For example, it may be used
for loss detection and recovery. In another example, it may be
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used to concatenate multiple small packets and reduce per packet
overhead.
4. GMA Encapsulation Methods
The GMA encapsulation protocol supports the following three
methods:
o Trailer-based IP Encapsulation (4.1)
o Header-based IP Encapsulation (4.2)
o (Header-based) non-IP Encapsulation (4.3)
Non-IP encapsulation MUST be used if the original IP packet is
IPv6.
Trailer-based IP encapsulation MUST be used if it is supported by
GMA endpoints and the original IP packet is IPv4.
Header-based encapsulation MUST be used if the trailer-based
method is not supported by either Client or Multi-Access Gateway.
In this case, if the adaptation layer, e.g., UDP tunnelling,
supports non-IP packet format, non-IP encapsulation MUST be used;
otherwise, header-based IP encapsulation MUST be used.
If non-IP encapsulation is configured, a GMA header MUST be
present in every packet. In comparison, if IP encapsulation is
configured, a GMA header or trailer may be added dynamically on
per-packet basis, and it indicates the presence of GMA header (or
trailer) to set the protocol type of the GMA PDU to "114" (see
Section 4.4).
The GMA endpoints MAY configure the GMA encapsulation method
through control signalling or pre-configuration. For example, the
"MX UP Setup Configuration Request" message as specified in Multi-
Access Management Service [MAMS] includes "MX Convergence Method
Parameters", which provides the list of parameters to configure
the convergence layer, and can be extended to indicate the GMA
encapsulation method.
GMA endpoint MUST discard a received packet and MAY log an error
to report the situation (although such error logging MUST be
subject to rate limits) under any of the following conditions:
. the GMA version number in the GMA header (or trailer) is not
understood or supported by the GMA endpoint
. a Flag bit in the GMA header (or trailer) not understood or
supported by the GMA endpoint is set to "1"
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4.1. Trailer-based IP Encapsulation
|<---------------------GMA PDU ----------------------->|
+------------------------------------------------------+
| IP hdr | IP payload | GMA Trailer |
+------------------------------------------------------+
|<------- GMA SDU (user payload)-------->|
Figure 3: GMA PDU Format with Trailer-based IP Encapsulation
This method SHALL NOT be used if the original IP packet (GMA SDU)
is IPv6.
Figure 3 shows the trailer-based IP encapsulation GMA PDU
(protocol data unit) format. 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 Protocol Type field in the IP header of the GMA PDU MUST be
changed to 114 (Any 0-Hop Protocol) (see Section 4.4) to indicate
the presence of the GMA trailer.
The following three IP header fields MUST be changed:
o IP Length: add the length of "GMA Trailer" to the length of
the original IP packet
o Time To Live (TTL): set to "1"
o IP checksum: recalculate after changing "Protocol Type", "TTL"
and "IP Length"
The GMA (Generic Multi-Access) trailer MUST consist of two
mandatory fields (the last 3 bytes): Next Header and Flags,
defined as follows:
o Next Header (1 Byte): the IP protocol type of the (first) SDU
in a PDU, and it stores the value before it was overwritten to
114.
o Flags (2 Bytes): Bit 0 is the most significant bit (MSB), and
Bit 15 is the least significant bit (LSB)
+ Checksum Present (bit 0): If the Checksum Present bit is set
to 1, then the Checksum field is present
+ Concatenation Present (bit 1): If the Concatenation Present
bit is set to 1, then the PDU carries multiple SDUs, and the
First SDU Length field is present
+ Connection ID Present (bit 2): If the Connection ID Present
bit is set to 1, then the Connection ID field is present
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+ Flow ID Present (bit 3): If the Flow ID Present bit is set
to 1, then the Flow ID field is present
+ Fragmentation Present (bit 4): 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
+ Delivery SN Present (bit 5): If the Delivery SN (Sequence
Number) Present bit is set to 1, then the Delivery SN field is
present and contains the valid information
+ Flow SN Present (bit 6): If the Flow SN Present bit is set
to 1, then the Sequence Number field is present
+ Timestamp Present (bit 7): If the Timestamp Present bit is
set to 1, then the Timestamp field is present
+ TTL Present (bit 8): If the TTL Present bit is set to 1,
then the TTL field is present
+ Reserved (bit 9-12): set to "0" and ignored on receipt
+ Version (bit 13~15): GMA version number, set to 0 for the
GMA encapsulation protocol specified in this document.
The Flags field is at the end of the PDU, and the Next Header
field is the second last. The Receiver SHOULD first decode the
Flags field to determine the length of the GMA trailer, and then
decode each optional field accordingly. The GMA (Generic Multi-
Access) trailer MAY consist of the following optional fields:
o Checksum (1 Byte): to contain the (one's complement) checksum
sum of all the 8 bits 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 First SDU Length (2 Bytes): the length of the first IP packet
in the PDU, only included if a PDU contains multiple IP
packets. This field is present only if the Concatenation
Present bit is set to one.
o Connection ID (1 Byte): an unsigned integer to identify the
anchor and delivery connection of the GMA PDU. This field is
present only if the Connection ID Present bit is set to one.
+ Anchor Connection ID (MSB 4 Bits): an unsigned integer to
identify the anchor connection
+ Delivery Connection ID (LSB 4 Bits): an unsigned integer to
identify the delivery connection
o Flow ID (1 Byte): an unsigned integer to identify the IP flow
that a PDU belongs to, for example Data Radio Bearer (DRB) ID
[LWIPEP] for a cellular (e.g., LTE) connection. This field is
present only if the Flow ID Present bit is set to one.
o Fragmentation Control (FC) (1 Byte): to provide necessary
information for re-assembly, only needed if a PDU carries
fragments. This field is present only if the Fragmentation
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Present bit is set to one. Please refer to section 5 for its
detailed format and usage.
o Delivery SN (1 Byte): an auto-incremented integer to indicate
the GMA PDU transmission order on a delivery connection.
Delivery SN is needed to measure packet loss of each delivery
connection and therefore generated per delivery connection per
flow. This field is present only if the Delivery SN Present
bit is set to one.
o Flow SN (3 Bytes): an auto-incremented integer to indicate the
GMA SDU (IP packet) order of a flow. Flow SN is needed for
retransmission, reordering, and fragmentation. It SHALL be
generated per flow. This field is present only if the Flow SN
Present bit is set to one.
o Timestamp (4 Bytes): to contain the current value of the
timestamp clock of the transmitter in the unit of 1
millisecond. This field is present only if the Timestamp
Present bit is set to one.
o TTL (1 Byte): to contain the TTL value of the original IP
header if the GMA SDU is IPv4, or the Hop-Limit value of the
IP header if the GMA SDU is IPv6. This field is present only
if the TTL Present bit is set to one.
Figure 4 shows the GMA trailer format with all the fields present,
and the order of the GMA control fields SHALL follow the bit order
in the Flags field. Note that the bits in the Flags field are
ordered with the first bit transmitted being bit 0 (MSB). All
fields are transmitted in regular network byte order and appear in
reverse order to their corresponding flag bits. If a flag bit is
clear, the corresponding optional field is absent.
For example, Bit 0 (the MSB) of the Flags field is the Checksum
Present bit, and the Checksum field is the last in the trailer
except of the two mandatory fields. Bit 1 is the Concatenation
present bit, and the FSL field is the second last.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TTL | Timestamp
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flow SN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delivery SN | FC | Flow ID | Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First SDU Length (FSL) | Checksum | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: GMA Trailer Format with all Optional Fields Present
4.2. Header-based IP Encapsulation
This method SHALL NOT be used if the original IP packet (GMA SDU)
is IPv6.
Figure 5 shows the header-based IP encapsulation format. Here, the
GMA header is inserted right after the IP header of the GMA SDU,
and the IP header fields of the GMA PDU MUST be changed the same
way as in trailered-based IP encapsulation.
+-----------------------------------------------+
|IP hdr | GMA Header | IP payload |
+-----------------------------------------------+
Figure 5: GMA PDU Format with Header-based IP Encapsulation
Figure 6 shows the GMA header format. In comparison to GMA
trailer, the only difference is that the Flags field is now in the
front so that the Receiver can first decode the Flags field to
determine the GMA header length.
"TTL" field MUST be included and the "TTL" bit in the GMA header
(or Trailer) MUST be set to 1 if (Trailer or Header based) IP
Encapsulation is used.
+------------------------------------------------------+
| Flags | other fields (TTL, Timestamp, Flow SN, etc.) |
+------------------------------------------------------+
Figure 6: GMA Header Format
4.3. (Header-based) non-IP Encapsulation
Figure 7 shows the header-based non-IP encapsulation format. Here,
"UDP Tunnelling" is configured at the MX adaptation layer. The
ports for "UDP Tunnelling" at Client are chosen from the Dynamic
Port range, and the ports for "UDP Tunnelling" at Multi-Access
Gateway are configured and provided to Client through additional
control messages, e.g., [MAMS].
"TTL", "FSL", and "Next Header" are no longer needed, and MUST not
be included. Moreover, the IP header fields of the GMA SDU remain
unchanged.
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+-------------------------------------------------------------+
| IP hdr | UDP hdr | GMA Header | IP hdr | IP payload |
+-------------------------------------------------------------+
|<------- GMA SDU------------>|
|<------------------- GMA PDU------------>|
Figure 7: GMA PDU Format with Non-IP Encapsulation
4.4. IP Protocol Identifier
As described in Section 4.1, IP encapsulated GMA PDUs are
indicated using the IP Protocol Type 114. This is designated and
recorded by IANA [IANA] to indicate "any 0-Hop Protocol". No
reference is given in the IANA registry for the definition of this
Protocol Type, and IANA has no record of why the assignment was
made or how it is used, although it was probably assigned before
1999 [IANA1999].
There is some risk associated with "re-using" Protocol Type 114
because there may be implementations of other protocols also using
this Protocol Type. However, because the protocol described in
this document is used only between adjacent devices specifically
configured for this purpose, the use of Protocol Type 114 should
be safe.
As described in Section 1.1, one of the purposes of the experiment
described in this document is to verify the safety of using this
Protocol Type. Deployments should be aware of the risk of a clash
with other uses of this Protocol Type.
5. Fragmentation
If the MTU size of the anchor connection (for GMA SDU) is
configured such that the corresponding GMA PDU length adding GMA
header (or trailer) and other overhead (UDP tunneling) MAY exceed
the MTU of a delivery connection, GMA endpoints MUST be configured
to support fragmentation through additional control messages
[MAMS].
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
o The maximum number of fragments per SDU is 2^7 (=128)
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The Fragmentation Control (FC) field in the GMA trailer (or
header) 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 Flow 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 n PDUs and copies
the content of the IP header fields from the long PDU into the IP
header of all the 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 114.
The data of the long SDU is divided into n 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 i-th portion of the data is placed in the
i-th PDU. The MF flag is set to "0" if it is the last fragment and
set to "1" otherwise. The FO field is set to i-1.
To assemble the fragments of a SDU, the receiver combines PDUs
that all have the same Flow SN. 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 GMA trailer (or header).
The first fragment will have the Fragment Offset set to "0", and
the last fragment will have the More-Fragments flag set to "0".
GMA fragmentation operates above the IP layer of individual access
connection (Wi-Fi, LTE) and between the two end points of
convergence layer. The convergence layer end points (client,
multi-access gateway) SHOULD obtain the MTU of individual
connection through either manual configuration or implementing
PMTUD as suggested in [RFC8900].
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6. 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.
If the (trailer or header based) IP encapsulation method is used,
the First SDU Length (FSL) field SHOULD be included in the GMA
trailer (or header) to indicate the length of the first SDU.
Otherwise, the FSL field SHOULD not be included.
+-----------------------------------------------------------+
|IP hdr| IP payload |IP hdr| IP payload | GMA Trailer |
+-----------------------------------------------------------+
Figure 8: Example of GMA PDU Format with Concatenation and
Trailer-based IP Encapsulation
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 8). The procedure continues till the PDU size reaches the
MTU of the delivery connection. If the FSL field is present, the
IP length field of the PDU SHOULD be updated to include all
concatenated SDUs and the trailer (or header), and the IP checksum
field SHOULD be recalculated if the packet is IPv4.
To disaggregate a PDU, if the (header or trailer based) IP
encapsulation method is used, the receiver first obtains the
length of the first SDU from the FSL field and decodes the first
SDU. 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 the non-IP encapsulation method (Figure 7) is used, the IP
header of the first SDU will not change during the encapsulation
process, and the receiver SHOULD obtain the length of the first
SDU directly from its IP header (Figure 9).
|<-------1st GMA SDU------------
+---------------------------------------------------------------+
| IP hdr | UDP hdr | GMA Header | IP hdr | IP payload |
+---------------------------------------------------------------+
| IP hdr | IP payload |
+-------------------------------------------+
-------->|<-------2nd GMA SDU--------------->
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Figure 9: Example of GMA PDU Format with Concatenation and Header-
based Non-IP (UDP) Encapsulation
If a PDU contains multiple SDUs, the Flow SN field is for the last
SDU, and the Flow 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.
GMA concatenation can be used for packing small packets of a
single application, e.g., TCP ACKs, or from multiple applications.
Notice that a single GMA flow may carry multiple application flows
(TCP, UDP, etc.).
GMA endpoint MUST NOT perform concatenation and fragmentation in a
single PDU. If a GMA PDU carries a fragmented SDU, it MUST NOT
carry any other (fragmented or whole) SDU.
7. Security Considerations
Security in a network using GMA should be relatively similar to
security in a normal IP network. GMA is unaware of IP or higher
layer end-to-end security as it carries the IP packets as opaque
payload. Deployers are encouraged to not consider that GMA adds
any form of security and to continue to use IP or higher layer
security as well as link-layer security.
The GMA protocol at the convergence sublayer is a 0-hop protocol
and 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. On the other hand, packet filtering requires either that
a firewall looks inside the GMA packet or that the filtering is
done on the GMA endpoints. In those environments in which this is
considered to be a security issue it may be desirable to terminate
the GMA operation at the firewall.
Local-only packet leak prevention (HL=0, TTL=1) SHOULD be on
preventing the leak of the local-only GMA PDUs outside the "local
domain" to the Internet or to another domain which could use the
same IP protocol type, i.e. 114.
8. IANA Considerations
This document makes no requests of IANA
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[GRE1] Dommety, G., "Key and Sequence Number Extensions to GRE",
<https://www.rfc-editor.org/info/rfc2890> .
9.2. Informative References
[MAMS] S. Kanugovi, F. Baboescu, J. Zhu, and S. Seo "Multi-Access
Management Services
(MAMS)https://tools.ietf.org/rfc/rfc8743.txt
[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.
[GRE2] RFC 8157, Huawei's GRE Tunnel Bonding Protocol, May 2017
[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan,
O., and F. Gont, "IP Fragmentation Considered Fragile",
BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.
[IANA1999]https://web.archive.org/web/19990203044112/http://www.is
i.edu:80/in-notes/iana/assignments/protocol-numbers
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[GCC] S. Holmer, et al. A Google Congestion Control Algorithm for
Real-Time Communication,
https://www.ietf.org/archive/id/draft-ietf-rmcat-gcc-
02.txt
[MPIP] L. Sun, et al. Multipath IP Routing on End Devices:
Motivation, Design, and
Performance, https://eeweb.engineering.nyu.edu/faculty/
yongliu/docs/MPIP_Tech.pdf
[GMAC] J. Zhu M. Zhang, UDP-based GMA Control
Protocol, https://www.ietf.org/archive/id/draft-zhu-
intarea-gma-control-00.txt
Authors' Addresses
Jing Zhu
Intel
Email: jing.z.zhu@intel.com
Satish Kanugovi
Nokia
Email: satish.k@nokia.com
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