Internet Engineering Task Force H. Seidel, Ed.
Internet-Draft BENOCS GmbH
Intended status: Informational October 19, 2015
Expires: April 21, 2016
ALTO map calculation from live network data
draft-seidel-alto-map-calculation-00
Abstract
This document describes a process to generate ALTO compliant
information from live network data.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Network Data Collection . . . . . . . . . . . . . . . . . . . 3
2.1. Topology Information . . . . . . . . . . . . . . . . . . 3
2.2. Routing Information . . . . . . . . . . . . . . . . . . . 4
2.3. Extended Information . . . . . . . . . . . . . . . . . . 4
2.4. Example Network . . . . . . . . . . . . . . . . . . . . . 5
2.5. Experiences . . . . . . . . . . . . . . . . . . . . . . . 7
3. Network Data Processing . . . . . . . . . . . . . . . . . . . 7
3.1. Topology Graph . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Router . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.2. Links . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Example Router R2 . . . . . . . . . . . . . . . . . . 8
3.2.2. Example Router R8 . . . . . . . . . . . . . . . . . . 9
3.2.3. Example Link . . . . . . . . . . . . . . . . . . . . 9
3.3. Experiences . . . . . . . . . . . . . . . . . . . . . . . 10
4. ALTO Map Calculation . . . . . . . . . . . . . . . . . . . . 10
4.1. Network Map Calculation . . . . . . . . . . . . . . . . . 11
4.1.1. Network Map Example . . . . . . . . . . . . . . . . . 11
4.1.2. Experiences . . . . . . . . . . . . . . . . . . . . . 12
4.2. Cost Map Calculation . . . . . . . . . . . . . . . . . . 13
4.2.1. Cost Map Example . . . . . . . . . . . . . . . . . . 13
4.2.2. Experiences . . . . . . . . . . . . . . . . . . . . . 14
5. Informative References . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Overview
The ALTO protocol is designed to export network information to
applications that need to select suitable endpoints among a wider set
of available ones. However, it does provide details about the
network information retrieval and processing.
This document describes a process to generate ALTO network and cost
maps from live network data and provides experience details about
that process in a large network.
The ALTO map generation process comprises three steps. The first
step is to gather information which is described in Section 2.
Subsequently the gathered data is processed which is described in
Section 3. The last section defines methods to generate ALTO network
and cost maps from the processed data.
In general it is not possible to gather detailed information about
the whole Internet since it is segmented in many networks and in most
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cases it is not possible to collect information across network
borders. Hence, information sources are limited to the own network.
2. Network Data Collection
The first step in the process of generating ALTO network and cost
maps from live network data is to gather the required information
from the network. This comprises at least topology and routing
information which contain details about present endpoints and their
interconnection and form the basic dataset. With this information it
is possible to compute paths between all known endpoints. The basic
dataset can be extended by many other information obtainable from the
network.
2.1. Topology Information
Topology information comprises details about routers and their
interconnection, also called links, within a network. Such
information are provided by various sources. The most prevalent
sources are interior gateway protocols (IGPs) which can be divided in
link-state (e.g. IS-IS, OSPF) and distance-vector protocols (RIP).
Most suitable are link-state protocols since every router propagates
its information throughout the whole network. Hence, it is possible
to obtain information about all routers and their neighbors from one
single router in the network. In contrast, distance-vector protocols
are less suitable since routing information is only shared among
neighbors. To obtain the whole topology with distance-vector routing
protocols it is necessary to retrieve routing information from every
router in the network.
Since IGPs lack of the possibility to easily steer traffic within the
network many network operators utilize MPLS to enable custom path
configuration. MPLS uses labels to identify configured paths. These
labelled paths create an overlay network on top of the actual network
forming its own virtual topology. Part of the MPLS architecture is
the Label Distribution Protocol (LDP) that is used to configure the
paths and therefore can be used to obtain MPLS topology information.
With the rise of software-defined networking (SDN) and its
abstraction of network management achieved by the decoupling of
network data and control plane network management became easier,
since the hardware does not require manual configuration anymore.
This is done by SDN controllers that relay routing information to the
switches and routers. So instead of gathering topology information
from the hardware within the network it can be fetched from SDN
controller.
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The data sources mentioned so far are only a subset of potential
topology sources and depending on the network type, (e.g. mobile,
satellite network) different hardware and protocols are in operation
to form and maintain the network.
2.2. Routing Information
Routing information comprises details about known endpoints and paths
in a network. In general there are two types of protocols, that
disseminate routing information on the Internet, interior gateway
(IGP) and exterior gateway protocol (EGP). While IGPs provide
details about endpoints and links within the own network, EGPs are
used to provide details about links to endpoints in foreign networks
outside of the operation scope of the own network. A path is
described by two endpoints and the traversed links. Routing
protocols assign metric values to links called link weights which
represents the cost to send data across a link. With the knowledge
about the link weights routing algorithms (e.g. Bellman-Ford)
calculate the path through the network for each source-destination
endpoint pair in the network.
The most widely-used routing protocols on the Internet are IS-IS,
OSPF and BGP. IS-IS and OSPF are IGPs and have already been
introduced in Section 2.1. BGP is an EGP based on the distance-
vector algorithm. As characteristic for distance-vector protocols,
it only shares routing information among neighbors. If no BGP route
reflector is present that collects routing information from all BGP
routers it is necessary to pick up that information directly from
each BGP router in the network. However, BGP is not only used as EGP
but also alongside IGPs (iBGP) to distribute known endpoints and the
corresponding metrics within a network.
In large real life network deployments such as ISP networks IGPs are
mainly used to disseminate topology information and link metrics.
Endpoint information such as subnets and attachment points are mostly
distributed by (i)BGP.
The previously mentioned SDN controller of a SDN is also a suitable
source for routing information. In general, as with topology details
the available routing information sources mainly depend on the
network type. However, our work focuses on networks using IS-IS or
OSPF as IGP and BGP as EGP.
2.3. Extended Information
Besides topology and routing information which are fundamental to
know how data between endpoints are exchanged, networks have a
multitude of other attributes about its state, condition and
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operation. That comprises but is not limited to attributes like link
utilization, bandwidth and delay, ingress/egress points of data flows
from/towards endpoints outside of the network up to the location of
nodes and endpoints. In general, extended information comprises all
information that a network provides which does not belong to topology
or routing. Typical sources are SNMP, Netflow or an operations
support system (OSS).
2.4. Example Network
Figure 1 depicts a network which is used to explain the steps carried
out in the course of this document. The network consists of nine
routers (R1 to R9) whereat two of them are border routers (R1 + R8)
connected to neighbored networks (AS 2 to AS 4). Furthermore, AS 4
is not directly connected to the local network but has AS 3 as
transit network. The links between the routers are point-to-point
connections, hence a /30 subnet is sufficient for each. These
connections also form the core network which we assigned the
100.1.1.0/24 subnet. This subnet is large enough to provide /30
subnets for all router interconnections. In addition to the core
network the local network also has five client networks attached to
five different routers (R2, R5, R6, R7 and R9). Each client network
is a /24 subnet with 100.1.10x.0 (x = [1..5]) as network address.
The example network utilizes two different routing protocols, one for
IGP and another for EGP routing. The used IGP is a link-state
protocol (IS-IS). The applied link weights are shown in Figure 2.
To obtain the topology and routing information from the network the
ALTO server must be connected directly to one of the routers
(R1...R9), Furthermore, the server must be enabled to communicate
with the router and vice versa.
The applied EGP in the network is the border gateway protocol (BGP),
which is used to route between autonomous systems (AS). So, BGP is
running on the two border routers R1 and R8. Furthermore, internal
BGP is used to propagate external as well as internal prefixes within
the network boundaries. Hence it is running on every router with an
attached client network (R2, R5, R6, R7 and R9). If no route
reflector is present it is necessary to fetch routes from each BGP
router separately. Otherwise, only one connection to route reflector
is sufficient to obtain all routes.
For monitoring purposes, SNMP is enabled on all routers within the
network. Thus, using SNMP an ALTO server is capable to obtain
several additional information about the state of the network. In
this example, utilization, latency and bandwidth information are
retrieved periodically via SNMP from the network components to get
and keep an up-to-date view on the network situation.
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+--------------+ +--------+ +--------+ +--------------+
|100.1.102.0/24+------+ R6 | | R7 +-----+100.1.103.0/24|
+--------------+ +----+---+ +----+---+ +--------------+
| |
+--------------+ | |
| AS 2 | | |
| 100.2.0.0/16 | | |
+-------+------+ | |
| | |
| | |
+---+----+ +----+---+ +----+---+ +--------------+
| R1 +--------+ R3 +------+ R5 |-----+100.1.104.0/24|
+---+----+ +----+---+ +----+---+ +--------------+
| \ / | |
| \ / | |
| \ / | | +--------------+
| \/ | | | AS 4 |
| /\ | | | 100.4.0.0/16 |
| / \ | | +------+-------+
| / \ | | |
| / \ | | |
+---+----+ +----+---+ +----+---+ +------+-------+
| R2 | | R4 | | R8 +-----+ AS 3 |
+---+----+ +----+---+ +----+---+ | 100.3.0.0/16 |
| | | +--------------+
| | |
| | |
+-------+------+ | +----+---+ +--------------+
|100.1.101.0/24| +----------+ R9 +-----+100.1.105.0/24|
+--------------+ +--------+ +--------------+
Figure 1: Example Network
R1 R2 R3 R4 R5 R6 R7 R8 R9
R1 0 15 15 20 - - - - -
R2 15 0 20 - - - - - -
R3 15 20 0 5 5 10 - - -
R4 20 - 5 0 5 - - - 20
R5 - - 5 5 0 - 10 10 -
R6 - - 10 - - 0 - - -
R7 - - - - 10 - 0 - -
R8 - - - - 10 - - 0 10
R9 - - - 20 - - - 10 0
Figure 2: Example Network Link Weights
In summary, the following information are collected from the network:
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IS-IS: topology, Link weights
BGP: prefixes, AS numbers, AS distances, metrics
SNMP: latency, utilization, bandwidth
2.5. Experiences
To be able to retrieve the information presented in this chapter we
implemented many of the mentioned protocols. While connecting to the
different data source we faced behavior that was different than we
anticipated from our interpretation of the corresponding protocol.
This includes behavior caused by older versions of the protocol
specification, a lax interpretation on the remote side or simply
incompatibility with the corresponding standard.
3. Network Data Processing
Due to the variety of data source available in today's network it is
necessary to aggregate the information and define a suitable data
model that can hold it efficiently and easily accessible. The most
suitable model is an annotated directed graph, since it perfectly
fits to represent topology and the attributes can be annotated at the
corresponding positions in the graph itself. More details about the
topology graph are provided in the following section.
3.1. Topology Graph
The topology graph is the data model that represents the surveyed
network. The node of the graph represents the routers in the network
while the edges stand for the links that connect the routers. Both
routers and links have a set of attributes that holds information
gathered from the network.
3.1.1. Router
The routers connect the different segments of the network. Each
router holds a basic set of information which are described in the
upcoming list:
ID: Unique ID within the network to identify the router.
Neighbor IDs: List of directly connected routers.
Endpoints: List of connected endpoints. The endpoints can also have
further attributes themselves depending on the network and address
type. Such potential attributes are costs for reaching the
endpoint from the router, AS numbers or AS distances. Be aware
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that endpoints can belong to more than one router, for example
when they are assigned to link interfaces.
In addition to the basic set many more attributes can be assigned to
router nodes. This mainly depends on the utilized data sources.
Such additional attributes can be geographic location, host name and/
or interface types, just to name a few.
3.1.2. Links
A link is a unidirectional connection between two routers. The basic
information set hold by a link is described in the following list:
Source ID: ID of the source router of the link.
Destination ID: ID of the destination router of the link.
Weight: The cost to cross the link defined by the used IGP.
Additional attributes that provide technical details and state
information can be assigned to links as well. The availability of
such additional attributes depends on the utilized data sources.
Such attributes can be characteristics like maximum bandwidth,
utilization or latency on the link as well as the link type. Even
the link cable color is a possible attribute, even though its sense
is doubtful.
3.2. Example
Picking up the example from Section 2.4 the example network shown in
Figure 1 already represents the layout of our internal network graph
where the routers R1 to R9 represent the nodes and the connections
between them are the links.
3.2.1. Example Router R2
ID: 2
Neighbor IDs: 1,3 (R1, R3)
Endpoints:
Endpoint: 100.1.101.0/24
Metric: 10 (default client subnet metric)
ASNumber: 1 (our own AS)
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ASDistance: 0
Host Name: R2
R2 has one directly attached IPv4 endpoint that belongs to its own
AS.
3.2.2. Example Router R8
ID: 8
Neighbor IDs: 5,9 (R5, R9)
Endpoints:
Endpoint: 100.3.0.0/16
Metric: 100
ASNumber: 3
ASDistance: 1
Endpoint: 100.4.0.0/16
Metric: 200
ASNumber: 4
ASDistance: 2
Host Name: R8
R8 has two attached IPv4 endpoints. The first one belongs to a
directly neighbored AS with AS number 3. So the AS distance from our
network to AS3 is 1. The second endpoint belongs to an AS (AS4) that
is no direct neighbor but directly connected to AS3. To reach
endpoints in AS4 it is necessary to cross AS3, which increases the AS
distance by one.
3.2.3. Example Link
The collectable link attributes are equal for all links and only
their values differ. The attributes utilization, bandwidth and
latency collected via SNMP where added to the existing example from
Section 2.4. Taking the links between R1 and R2 as example, we get
the following link attributes:
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R1->R2:
Source ID: 1
Destination ID: 2
Weight: 15
Bandwidth: 10Gbit/s
Utilization: 0.1
Latency: 2ms
R2->R1:
Source ID: 2
Destination ID: 1
Weight: 15
Bandwidth: 10Gbit/s
Utilization: 0.55
Latency: 5ms
Since the values for utilization and latency are very volatile the
presented values are only exemplary.
3.3. Experiences
Processing network information is very complex with a high demand in
resources. Gathering information from an autonomous system connected
to Internet means the software and machine must be able to store and
process hundreds of thousands of prefixes, several hundreds of
megabytes of Netflow information per minute, information from
hundreds of routers and attributes of thousands of links. Hence, a
lot of disk memory, RAM and CPU cycles as well as efficient
algorithms are required to process the information as they come in.
4. ALTO Map Calculation
The goal of the ALTO map calculation process is to get from the graph
presentation of the network as described in Section 3 to a coarse-
grained matrix presentation. The first step is to generate the
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network map. Only after the network map has been generated it is
possible to compute the cost map since it relies on the network map.
4.1. Network Map Calculation
To generate an ALTO network map a grouping function is required. A
grouping function processes information from the network graph to
group endpoints into PIDs. The way of grouping is manifold and
algorithm can utilize all information provided by the network graph
to perform the grouping. The functions may omit certain endpoints to
simplify the map or to hide details about the network that are not
intended to be published with the resulting ALTO network map.
For IP endpoints which is either an IP (version 4 or version 6)
address or prefix, [RFC7285] requires the use of longest-prefix
matching algorithm (Section 5.2.4.3 [RFC1812]) to map IPs to PIDs.
This requirement yields the constraints that every IP must be mapped
to a PID and that the same prefix or address is not mapped to more
than one PID. To meet the first constraint every calculated map must
provide a default PID that contains the prefixes 0.0.0.0/0 for IPv4
and ::/0 for IPv6. Both prefixes cover their entire address space
and if no other PID matches an IP endpoint the default PID will. The
second constraint must be met by the grouping function since it
assigns endpoints to PIDs, so in case of collision the grouping
function must decide which PID get the endpoint. Be aware that these
or even other constraints may apply to other endpoint types depending
on the used matching algorithm.
4.1.1. Network Map Example
A simple example on how such grouping can work is to group per host
name from our network graph. Each router host name is the name for a
PID and and the attached endpoints are the member endpoints of the
corresponding router PID. Additionally backbone prefixes should not
appear in the map so they are filtered out. The following table
shows the resulting ALTO network map based on the example network
from Section 2.4:
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PID | Endpoints
---------+-----------------------------------
R1 | 100.2.0.0/16
R2 | 100.1.101.0/24
R5 | 100.1.104.0/24
R6 | 100.1.102.0/24
R7 | 100.1.103.0/24
R8 | 100.3.0.0/16, 100.4.0.0/16
R9 | 100.1.105.0/24
default | 0.0.0.0/0, ::/0
Figure 3: Example ALTO Network Map
Since router R3 and R4 have no endpoints assigned they are not
represented in the network map. Furthermore, as previously mentioned
the "default" PID was added to represent all endpoints that are not
part of the example network.
4.1.2. Experiences
Large IP based networks consist of hundreds of thousands of prefixes
which are mapped to PIDs in the process of network map calculation.
As a result, network maps get very large (up to tens of megabytes).
However, depending on the design of the network and the chosen
grouping function the calculated network maps contains redundancy
that can be removed. There are at least two ways to reduce the size
by removing redundancy.
First, Adjacent IP prefixes can be merged. When a PID has two
adjacent prefix entries it can merge them together to one larger
prefix. It is mandatory that both prefixes are in the same PID.
However, it cannot be ruled out that the large prefix is assigned to
another PID. This MUST BE checked and it is up to the grouping
function whether it merges the prefixes and removes the larger prefix
from the other PID or not. A simple example, when a PID comprises
the prefixes 192.168.0.0/24 and 192.168.1.0/24 it can easily merge
them to 192.168.0.0/23.
Second, a prefix and its next-longer-prefix match are in the same
PID. In this case, the smaller prefix can simply be removed since it
is redundant for obvious reasons. A simple example, a PID comprises
the prefixes 192.168.0.0/22 and 192.168.1.0/24 and the /22 is the
next-longer prefix match of the /24, the /24 prefix can simply be
removed. In contrast, if another PID contains the 192.168.0.0/23
prefix 192.168.1.0/24 cannot be removed since the next-longer prefix
is not in the same PID anymore.
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4.2. Cost Map Calculation
After successfully creating the network map, the next step is to
calculate the costs between the PIDs, which forms the cost map.
Those costs are calculated by cost functions which use the
information provided by the network graph described in Section 3.
Cost function calculate values unidirectional which means that it is
necessary to compute the costs from every PID to every PID. In
general, it is possible to use all available information in the
network graph to compute the costs. In case a PID contains more than
one IP address or subnet the cost function calculates a set of cost
values for each source/destination IP pair. In that case a tie-
breaker function is required which decides the resulting cost value.
Such tie-breaker can be simple functions such as minimum, maximum or
average value.
Be aware, no matter what metric the cost function is using, the path
from source to destination is defined by the minimum path weight.
The path weight is the sum of link weights of all traversed links.
Usually, the path is determined with the Bellman-Ford or Dijkstra
algorithm. Hence, the cost function must first determine the path
before it can determine any other metric value. As a result, the
metric value can differ from the expectation where the path with the
shortest path weight was not considered. But it is also possible
that more than one path with the same minimum path weight exist,
which means it is not entirely clear which path is going to be
selected by the network. Hence, a tie-breaker similar to the one
used to resolve costs for PIDs with multiple endpoints is necessary.
An important note is that [RFC7285] does not require cost maps to
provide costs for every PID pair, so if no path cost can be
calculated for a certain pair the corresponding field in the cost map
is left out. Administrators MAY also not want to provide cost values
for other PID pairs for arbitrary reasons. Such pairs MAY BE defined
before the cost calculation is performed and should be respected in
the calculation process even though cost values are computable.
There is an active internet draft [I-D.yang-alto-path-vector] that
proposes vector based cost maps rather then the current point-to-
point matrix. A vector representation can reduce the required
computation to generate cost maps from a directed graph as proposed
in Section 3.
4.2.1. Cost Map Example
Based on the network map generated in Section 4.1.1 it is possible to
calculate the cost maps. In this example the chosen metric for the
cost map is the minimum number of hops necessary to get from source
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to destination PID. Our chosen tie-breaker selects the minimum hops
when more than one value is returned by the cost function.
PID | default | R1 | R2 | R5 | R6 | R7 | R8 | R9 |
--------+---------+-----+-----+-----+-----+-----+-----+-----|
default | x | x | x | x | x | x | x | x |
R1 | x | 0 | 2 | 3 | 3 | 4 | 4 | 3 |
R2 | x | 2 | 0 | 3 | 3 | 4 | 4 | 4 |
R5 | x | 3 | 3 | 0 | 3 | 2 | 2 | 3 |
R6 | x | 3 | 3 | 3 | 0 | 4 | 4 | 4 |
R7 | x | 4 | 4 | 2 | 4 | 0 | 3 | 4 |
R8 | x | 4 | 4 | 2 | 4 | 3 | 0 | 2 |
R9 | x | 3 | 4 | 3 | 4 | 4 | 2 | 0 |
Figure 4: Example ALTO Hopcount Cost Map
Interesting to mention is, that R1<->R9 has several paths with equal
path weights. The paths R1->R3->R5->R8->R9, R1->R3->R4->R9 and
R1->R4->R9 all have a path weight of 40. Due to our minimum value
tie-breaker 3 hops for the path R1->R4->R9 is chosen as value.
Furthermore, since the "default" PID is sort of virtual PID with no
endpoints that are part of the example network no cost values are
calculated for other PIDs from or towards it.
4.2.2. Experiences
The major challenge we encountered with cost map calculations was the
vast amount of CPU cycles that where required to calculate the costs
in large networks. The issue was that the costs were calculated
between the endpoints of each source-destination PID pair. However,
very often several to many endpoints of a PID are attached to the
same node, so the same path cost is calculated several times. This
is clearly inefficient. So instead, we looked up the routers the
endpoints of each PID are connected to in our network graph and
calculated cost map based on the costs between the routers.
5. Informative References
[I-D.yang-alto-path-vector]
Bernstein, G., Lee, Y., Roome, W., Scharf, M., and Y.
Yang, "ALTO Extension: Abstract Path Vector as a Cost
Mode", draft-yang-alto-path-vector-01, July 2015.
[RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "A
Simple Network Management Protocol (SNMP)", RFC 1157, May
1990.
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[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC2328] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7285] Almi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
Traffic Optimization (ALTO) Protocol", RFC 7285, September
2014.
[RFC7607] Kumari, W., Bush, R., Schiller, H., and K. Patel,
"Codification of AS 0 Processing", RFC 7607, August 2015.
Author's Address
Hans Seidel (editor)
BENOCS GmbH
Email: hseidel@benocs.com
Seidel Expires April 21, 2016 [Page 15]