ippm H. Song, Ed.
Internet-Draft T. Zhou
Intended status: Experimental Huawei
Expires: December 29, 2017 June 27, 2017
On Scalability of In-situ OAM
draft-song-ippm-ioam-scalability-01
Abstract
This document describes several potential scalability issues when
implementing in-situ OAM based on the current in-situ OAM documents
and proposes the corresponding solutions and modifications to the
current in-situ OAM specification. Specifically, we extend in-situ
OAM to support more standard tracing data than is currently defined
and add new features to avoid limitations on MTU, bandwidth,
forwarding path length, and node processing capability. We provide
use cases to motivate our proposal and base the changes on the
current in-situ OAM header format specification.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Motivation for Better iOAM Scalability . . . . . . . . . . . 2
2.1. Support Data Type Extensions . . . . . . . . . . . . . . 3
2.1.1. Motivating Use Cases . . . . . . . . . . . . . . . . 3
2.2. Cope with Packet Size Limitation . . . . . . . . . . . . 4
2.2.1. Motivating Use Cases . . . . . . . . . . . . . . . . 4
2.3. Adapt to Node Processing Capability . . . . . . . . . . . 4
2.3.1. Motivating Use Cases . . . . . . . . . . . . . . . . 5
3. Scalable Data Type Extension . . . . . . . . . . . . . . . . 5
3.1. Data Type Bitmap . . . . . . . . . . . . . . . . . . . . 5
3.2. Scalable Data Type Extension Use Cases . . . . . . . . . 6
3.3. Consideration for Data Packing . . . . . . . . . . . . . 7
3.4. Other Data Extension Possibilities . . . . . . . . . . . 7
4. Segment In-situ OAM . . . . . . . . . . . . . . . . . . . . . 7
4.1. Segment and Hops . . . . . . . . . . . . . . . . . . . . 7
4.2. Considerations for Data Handling . . . . . . . . . . . . 8
4.3. Segment iOAM Use Cases . . . . . . . . . . . . . . . . . 8
5. In-situ OAM Sampling and Data Validation . . . . . . . . . . 9
5.1. Valid Node Bitmap and Valid Data Bitmap . . . . . . . . . 9
5.2. iOAM Sampling and Data Validation Use Cases . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 11
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
In-situ OAM (iOAM) [I-D.brockners-inband-oam-requirements] records
OAM information within user packets while the packets traverse a
network. The data types and data formats for in-situ OAM data
records have been defined in [I-D.brockners-inband-oam-data]. We
identify several scalability issues for implementing the current iOAM
specification and propose solutions in this draft.
2. Motivation for Better iOAM Scalability
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2.1. Support Data Type Extensions
Currently 11 data types and associated formats (including wide format
and short format of the same data) are defined in
[I-D.brockners-inband-oam-data] . The presence of data is indicated
by a 16-bit bitmap in the "OAM-Trace-Type" field.
In the current specification only five bits are left to identify new
data types. Moreover, some data is forced to be bundled together as
a single unit to save bitmap space and pack data to the ideal size
(e.g., the hop limit and the node id are bundled, and the ingress
interface id and the egress interface id are bundled), regardless of
the fact that an application may only ask for a part of the data.
Last but not the least, each data is forced to be 4-byte aligned for
easier access, resulting in waste of header space in many cases.
Since the data plane bandwidth, the data plane packet processing, and
the management plane data handling are all precious yet scarce
resource, the scheme should strive to be simple and precise. The
application should be able to control the exact type and format of
data it needs to collect and analyze. It is conceivable that more
types of data may be introduced in the future. However, the current
scheme cannot support it after all the bits in the bitmap are used
up.
Currently, bit 7 is used to indicate the presence of variable length
opaque state snapshot data. While this data field can be used to
store arbitrary data, the data is difficult to be standardized and
another schema is needed to decode the data, which may lead to low
data plane performance.
2.1.1. Motivating Use Cases
When a flow traverses a series of middleboxes (e.g., Firewall, NAT,
and load balancer), its identity (e.g., the 5-tuple) is often
altered, which makes the OAM system lose track of the flow trace. In
this case, we may want to copy some of the original packet header
fields into the iOAM header so the original flow can be identified at
any point of the network.
In wireless, mobile, and optical network environments, some physical
data associated with a flow (e.g., power, temperature, signal
strength, GPS location) need to be collected to monitor the service
performance.
Both cases require new iOAM data types. More examples are listed in
Section 3.2.
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2.2. Cope with Packet Size Limitation
The total size of data is limited by the MTU. When the number of
required data types is large and the forwarding path length is long,
it is possible that there is not enough space in the iOAM header to
save the data. The current proposal is to label the overflow status
and stop adding new node data to the packet, leading to loss of
information.
Even if the header has enough space to hold the iOAM data, the
overhead may be too large and consume too much bandwidth. For
example, if we assume moderate 20 bytes of data per node, a path with
length of 10 will need 200 bytes to hold the data. This will inflate
small 64-byte packets by more than four times. Even for the largest
packet size (e.g., 1500 bytes), the overhead (>10%) is not
negligible. Therefore, we need to limit the iOAM data overhead
without sacrificing the data collection capability.
Here we have another interesting related issue. Packets can be
dropped anywhere in a network for various reasons. If we can only
collect iOAM data at the path end, we lose all data from the dropped
packets and have no idea where the packets are dropped. This defies
the purpose of iOAM and makes those iOAM-enabled nodes work in vain.
2.2.1. Motivating Use Cases
Some use cases are described in Section 4.3.
2.3. Adapt to Node Processing Capability
iOAM can designate the flow to add the iOAM header and collect data
on the flow forwarding path. The flow can have arbitrary
granularity. However, processing the data can be a heavy burden for
the network nodes, especially when some data needs to be calculated
by the node (e.g., the transit delay). If the flow traffic is heavy,
the node may not be able to handle the iOAM processing so many
performance issues may occur, such as long latency and packet drop.
Although it is good for the OAM applications to gain the detailed
information on every packet at every node, in many cases, such
information is often repetitive and redundant. The large quantity of
data would also burden the management plane which needs to collect
and stream the data for analytics. It is also possible that some
nodes cannot provide the requested data at all or are unwilling to
provide some data for security or privacy concerns. So a trade-off
is needed to balance the performance impact and the data availability
and completeness.
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2.3.1. Motivating Use Cases
To minimize the network impact, a network operator decides to collect
the iOAM data only for initial and last flow packets (e.g., TCP
packets with SYN, FIN, and RST flags).
A head node alternates two iOAM headers with each requesting a subset
of iOAM data. Hence, each node on the flow path only needs to handle
partial data. The requests can be balanced without exhausting the
network nodes.
A node is temporarily under heavy traffic load. It is in danger of
dropping packets if it tries to satisfy all the iOAM data requests.
In this case, it would rather deny some requests than drop user
traffic.
More examples are listed in Section 5.2.
3. Scalable Data Type Extension
Based on the observation in Section 2.1, we propose a method for data
type encoding which can solve the current limitation and address
future data requirements.
3.1. Data Type Bitmap
Bitmap is simple and efficient data structure for high performance
data plane implementation. The base bitmap size is kept to be 16
bits. We use one bit to indicate a single type of data in a single
format. The last bit in the bitmap (i.e., bit 15), if set, is used
to indicate the presence of the next data type bitmap, which is 32
bits long. In the second bitmap, bit 31 is again reserved to
indicate a third bitmap, and so on. With each extra bitmap, 31 more
data types can be defined.
Figure 1 shows an example of the in-situ OAM header format with two
extended OAM trace type fields. Except the OAM Trace Type fields,
all other fields remain the same as defined in
[I-D.brockners-inband-oam-data].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Base OAM Trace Type |1| Length Field | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended OAM Trace Type 1 |1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended OAM Trace Type 2 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Node Data List [] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended OAM Trace Type Header Format
The specification of the Base OAM Trace Type is the same as the OAM
Trace Type in [I-D.brockners-inband-oam-data] except the last bit,
which is defined as follows:
o Bit 15: When set indicates presence of next bit map.
The OAM trace type fields are labeled as Base OAM Trace Type,
Extended OAM Trace Type 1, Extended OAM Trace Type 2, and so on. The
Base OAM Trace Type is always present. If no data type is asked by
the application in Extended OAM Trace Type n and beyond, then the
last bit in the previous bitmap is set to 1 and these extended fields
are not included in the header. On the other hand, to eliminate
ambiguity, if any data is asked for by the application in Extended
OAM Trace Type n, then Extended OAM Trace Type 1 to (n-1) must be
included in the header, even though no data type in these bitmaps are
needed (i.e., all zero bitmap except the last bit).
The actual data in a node is packed together in the same order as
listed in the OAM Trace Type bitmap. Each node is padded to be the
multiple of 4 bytes.
3.2. Scalable Data Type Extension Use Cases
New types of data can be potentially added and standardized, which
demand new bits allocated in the OAM Trace Type bitmaps. Some
examples are listed here.
o Metered flow bandwidth.
o Time gap between two consecutive flow packets.
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o Remaining time budget to the packet delivery deadline.
o Buffer occupancy on the Node.
o Queue depth on each level of hierarchical QoS queues.
o Packet jitter at the Node.
o Current packet IP addresses.
o Current packet port numbers.
o Other node statistics.
3.3. Consideration for Data Packing
The length of each data must be the multiple of 2 bytes. However,
allowing different data type to have different length, while
efficient in storage, makes data alignment and packing difficult.
If we can define the maximum number of data types that can be carried
per packet, the offset of each data in the node can be pre-calculated
and carried in the iOAM header. The overhead can be justified by the
overall space saving of the node data list. Otherwise, each data's
offset in the node must be calculated in each device, with the help
of a table which stores the size of each data type. We can also
arrange the bitmap to reflect the data availability order in the
system (e.g., the bit for egress_if_id must be after the bit for
ingress_if_id), so in a pipeline-based system, the required data can
be packed one after one.
3.4. Other Data Extension Possibilities
Bitmap is simple and support parallel processing in hardware,
however, it is not the only option to support data type extension.
For example, cascaded TLV can be used to support arbitrary number of
new data types.
4. Segment In-situ OAM
Based on the observation in Section 2.2, we propose a method to limit
the size of the node data list.
4.1. Segment and Hops
A hop is a node on a flow's forwarding path which is capable of
processing iOAM data. A segment is a fixed number hops on a flow's
forwarding path. While working in the "per hop" mode, the segment
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size (SSize) and the remaining hops (RHop), is added to the iOAM
header at the edge. Initially, RHop is equal to SSize. At each hop,
if RH is not zero, the node data is added to the node data list at
the corresponding location and then RH is decremented by 1. If RH is
equal to 0 when receiving the packet, the node needs to remove (in
incremental trace option) or clear (in pre-allocated trace option)
the iOAM node data list and reset RHop to SSize. Then the node will
add its data to the node data list as if it is the edge node.
Figure 2 shows the proposed in-situ OAM header format. The last bit
(bit 31) in the Flags field is used to indicate the current header is
a segment iOAM header. In this context, the third byte of the first
word is partitioned into two 4-bit piece. The first piece is used to
save the segment size and the second piece is used to save the
remaining hops. This limits the maximum segment size to 15.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Base OAM Trace Type |0| SSize | RHop | Flags |1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Node Data List [] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Segment iOAM Header Format
4.2. Considerations for Data Handling
At any hop when RHop is equal to 0, the node data list is copied from
the iOAM header. The data can be encapsulated and reported to the
controller or the edge node as configured. The encapsulation and
report method is beyond the scope of this draft but should be comply
with the method used by the iOAM edge node.
The actual size of the last segment may not be equal to SSize but
this is not a problem.
4.3. Segment iOAM Use Cases
Segment iOAM is necessary in the following example scenarios:
o Segment iOAM can be used to detect at which segment the flow
packet is dropped. If the SSize is set to 1, then the exact drop
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node can be identified. The iOAM data before the dropping point
is also retained.
o The path MTU allows to add at most k node data in the list to
avoid fragmentation. Therefore SSize is set to k and at each hop
where RHop is 0, the node data list is retrieved and sent in a
standalone packet.
o A flow contains mainly short packets and travels a long path. It
would be inefficient to keep a large node data list in the packet
so the network bandwidth utilization rate is low. In this case,
segment iOAM can be used to limit the ratio of the iOAM data to
the flow packet payload.
o The network allows at most n bytes budget for the iOAM data.
There is a tradeoff between the number of data types that can be
collected and the number of hops for data collecting. The segment
size is therefore necessary to meet the application's data
requirement (i.e., SSize * Node Data Size < n).
5. In-situ OAM Sampling and Data Validation
Based on the observation in Section 1.3, the source edge node should
be able to define either the period or the probability to add the
iOAM header to the selected flow packet. In this way, only a subset
of the flow/sec packets would carry the OAM data, which not only
reduces the overall iOAM data quantity but also reduces the
processing work load of the network nodes.
5.1. Valid Node Bitmap and Valid Data Bitmap
It is possible that even an iOAM capable node will not add data to
the node data list as requested. In some cases, a node can be too
busy to handle the data request or some types of the requested data
is not available. Therefore, we propose to add two bitmaps, a valid
node bitmap and a valid data bit, to the iOAM specification.
The Node Valid Bitmap is inserted before the Node Data List as shown
in Figure 3. Each bit in the bitmap corresponds to a hop on the
packet's forwarding path. The bits are listed in the same order as
the hop on the packet's forwarding path. The bitmap is cleared to
all zero at first. If a hop can add data to the Node Data List, the
corresponding bit in Node Valid Bitmap is set to 1. The bit location
for a hop can be calculated from the length field (e.g, the bit index
is equal to SSize-RHop).The valid node data items in the node data
list is equal to the number of 1's in the Node Valid Bitmap.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Base OAM Trace Type |0| Length Field | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Valid Node Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Node Data List [] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Segment iOAM Header Format
For each node data in the node data list, a Valid Data Bitmap is
added before the node data. The number of bits in the Valid Data
Bitmap is equal to the number of 1's in the OAM Trace Type bitmaps
(excluding the next trace type bitmap indicator bits). When the bit
is set, the corresponding data is valid in the node; otherwise, the
corresponding data is invalid so the management plane should ignore
it after the data is collected.
The size of the bitmap can be padded to two or four bytes, which
allow up to 16 or 32 types of data to be included in a node.
5.2. iOAM Sampling and Data Validation Use Cases
We give some examples to show the usefulness of in-situ OAM sampling
and data validation features.
o An application needs to track a flow's forwarding path and knows
the path will not change frequently, so it sets a low sampling
rate to periodically insert the iOAM header to request the node
ID.
o In a heterogeneous data plane, some nodes support to provide data
x but the other nodes do not support it. However, an application
is still interested in collecting data x if available. In this
case, iOAM header can still be configured to ask for data x but
the nodes that cannot provide the data simply invalidates it by
resetting the corresponding bit in the valid data bitmap.
o Multiple sampling rate and multiple data request schema can be
defined for a flow based on applications requirements and the data
property, so for a flow packet, there can be no iOAM header or
different iOAM headers. The node does not need to process all
data all the time.
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o For security reason, a node decides to not participate in the iOAM
data collection. While it processes the other iOAM header fields
as usual, it does not set the node valid bit in the Node Valid
Bitmap and add node data to the Node Data List.
6. Security Considerations
There is no extra security considerations beyond those have been
identified by in-situ OAM protocol.
7. IANA Considerations
This memo includes no request to IANA.
8. Acknowledgments
We would like to thank Frank Brockners and Carlos Pignataro for
helpful comments and suggestions.
9. Contributors
The document is inspired by numerous discussions with James N.
Guichard. He also provided significant comments and suggestions to
help improve this document.
10. Informative References
[I-D.brockners-inband-oam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., and R. <>, "Data Formats for In-situ OAM", draft-
brockners-inband-oam-data-02 (work in progress), October
2016.
[I-D.brockners-inband-oam-requirements]
Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
Gredler, H., Leddy, J., Youell, S., Mozes, D., Mizrahi,
T., <>, P., and r. remy@barefootnetworks.com,
"Requirements for In-situ OAM", draft-brockners-inband-
oam-requirements-02 (work in progress), October 2016.
Authors' Addresses
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Haoyu Song (editor)
Huawei
2330 Central Expressway
Santa Clara, 95050
USA
Email: haoyu.song@huawei.com
Tianran Zhou
Huawei
156 Beiqing Road
Beijing, 100095
P.R. China
Email: zhoutianran@huawei.com
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