ippm H. Song, Ed.
Internet-Draft T. Zhou
Intended status: Standards Track Huawei
Expires: April 20, 2018 October 17, 2017
Control In-situ OAM Overhead with Segment IOAM
draft-song-ippm-segment-ioam-00
Abstract
This document describes a proposal to segment an in-situ OAM domain
in order to control the iOAM data overhead, adapt to the path MTU
limitations, and enable new applications. We provide use cases to
motivate our proposal and base the necessary modifications on the
current in-situ OAM header format specification.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Segment In-situ OAM . . . . . . . . . . . . . . . . . . . . . 3
2.1. Segment and Hops . . . . . . . . . . . . . . . . . . . . 3
2.2. Considerations for Data Handling . . . . . . . . . . . . 4
2.3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
6. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Informative References . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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.ietf-ippm-ioam-data].
iOAM may incur significant overhead on user packets. The overhead
includes the iOAM header and the node data list for each network
element.
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.
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2. Segment In-situ OAM
Based on the observation in Section 1, we propose a method to limit
the size of the node data list.
2.1. 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" trace mode, the
segment 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.
The stripped iOAM data at the segement edge can be immediately
exported to a collector.
Figure 1 shows the proposed in-situ OAM header format. The bit 23 in
the Flags field is used to indicate the current header is a segment
iOAM header. In this context, the last octet in the iOAM header is
partitioned into two 4-bit nibbles. The first nibble (SSize) is used
to save the segment size and the second nibble (RHop) 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 |NodeLen|Flags|1| SSize | RHop |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Node Data List [] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Segment iOAM Header Format
In the special case when SSize is set to 0, no data will be recorded
in the node data list. The requested data listed in the OAM Trace
Type will be immediately exported to the collector. This way the
iOAM overhead is minimized.
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2.2. 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.
2.3. 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
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).
3. Security Considerations
There is no extra security considerations beyond those have been
identified by in-situ OAM protocol.
4. IANA Considerations
This memo includes no request to IANA.
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5. Acknowledgments
We would like to thank Frank Brockners, Carlos Pignataro, and Shwetha
Bhandari for helpful comments and suggestions.
6. Contributors
The document is inspired by numerous discussions with James N.
Guichard. He also provided significant comments and suggestions to
help improve this document.
7. Informative References
[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.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., Chang, R., and d. daniel.bernier@bell.ca, "Data Fields
for In-situ OAM", draft-ietf-ippm-ioam-data-00 (work in
progress), September 2017.
Authors' Addresses
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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