Commit 73e36a1c authored by uvkjt's avatar uvkjt
Browse files

Updates for Autonomous Systems

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\chapter{Telematik}
Da die Vorlesungsfolien auf Englisch sind, ist dieser Teil der Zusammenfassung auf Englisch.
Da die Vorlesungsfolien auf Englisch sind, ist dieser Teil der Zusammenfassung auf Englisch. \par
Examples usually point to a slide of the lecture. They are marked with a \folie{F\textit{ff}S\textit{ss}} where \textit{ff} equals the slide number and \textit{ss} equals the slide file number (meaning the big file where all the slides are stored). In ilias the slide file numbering starts from 1, with 1 being the introduction and 2 the Router slides.
\section{Erinnerung aus Rechnernetze}
......@@ -132,7 +133,7 @@ Instead of using and address to find the port as in RAM, CAM uses a piece of dat
\subsubsection{Ternary CAM}
This allows LPM because the CAM registers can use Don't Care States, to fulfill the needed maximum length of a prefix. This practically means a sort of a multibit trie. The order of the elements in the TCAM is, longest prefix at the top and the shortest at the bottom. This way when a matched prefix is found, the first matching one in the list is the more precise, as it is longer. \par
TCAMs are very fast at a 1 lookup per clock cycle, but it has high energy demand and updates are really slow, as all registers need to be correctly reordered, otherwise the first match might not be the most precise. All in all, not scalable.
\section{Router Architecture}
\subsection{Router Architecture}
A router has 3 basic components:
\begin{itemize}
\item Network Interfaces (Realizes functions of L1 and L2)
......@@ -155,3 +156,42 @@ The way a router is designed, a packet blocking situation can occur, where $>1$
\item \textbf{Backpressure} \\ Signal overload to input to reduce load
\item \textbf{Parallel switch fabric} \\ Multiple packet served at the same time to output port. But requires higher access speed to cope up the feed size from parallelity.
\end{itemize}
\section{Internet Routing}
\begin{prereq}
Routing determines the complete path a packet takes and is part of the control plane in the router design. \par
A \textbf{routing table}, in contrast to the forwarding table, \textit{maps an ip address to the next destination IP prefix hop}, whereas the forwarding table\textit{ mapped an IP address to an output port}, which corresponded to a IP prefix.\par
A routing \textbf{metric} is simply a "cost" which is taken from the routing protocol into account to make a routing decision. An example would be latency, data rate, etc. \par
A routing \textbf{policy} is a set of rules that routing decisions should follow.
\end{prereq}
Currently the most commonly used routing method is the \textit{Distributed Adaptive Routing}. Every router contains an instance of the routing protocol and its routers communicate with each other using routing messages to adapt to the network situation. A network is modeled as a graph, with every node being a router and each edge the metric/cost.
\subsection{Autonomous Systems - \textit{AS}}
\subsubsection{AS Structure}
The internet takes shape as a big network of little networks inside called \textit{Autonomous Systems}. Example on \folie{F12S3}. Therefore 2 routing protocols are used:
\begin{itemize}
\item \textit{Interior Gateway Protocol} (IGP) which is the routing protocol used inside an AS.
\item \textit{Exterior Gateway Protocol} (EGP) which is the routing protocol used between different AS.
\end{itemize}
A single AS is identified by a 32 bit number called the ASN (which is allocated from IANA) and appears as a single entity to the outside. Its not perfectly scalable as the routing information and computation overhead increases with each new AS, as there are more routing messages that need to be calculated but its good because it guarantees operator autonomy meaning different AS-es can have different IGPs and internal structure is hidden. A real world AS subdivision on \folie{F17S3}. \par
An AS is classified in 3 roles:
\begin{enumerate}
\item \textbf{Stub AS} \\ Usually allocated for a small organization/enterprise and connected to one provider. Not for transit.
\item \textbf{Multihomed AS} \\ Allocated for large enterprises and connected to multiple providers. Not for transit.
\item \textbf{Transit AS} \\ The provider. For transit and therefore global scope.
\end{enumerate}
With this classification, numbered tiers can appear. For example Transit AS are of tier 1, and lower AS of a lower tier. Example on \folie{F19S3}.
\subsubsection{AS Reachability}
There are 2 basic reachability concepts. You either use a \textbf{transit}, by paying for connectivity or you can \textbf{peer} to directly connect to the another AS (which is typically of same tier). To connect means to establish a path to other AS-es in the Internet. In the case of transit reachability, this would mean connecting to a transit AS, and paying them for requesting a path to a destination by having them find and deliver the content to your AS. Good graphic example on \folie{F24S3}. In the case of peering, you establish a direct connection to the other AS and communicate only with it. This way the only cost, is that of the infrastructure.\par
There are 2 forms of peering:
\begin{enumerate}
\item \textbf{Private} \\ You directly connect to all AS-es (which would form a big mesh if there are too many AS-es and two AS-es can be geographically far away, which is bad)
\item \textbf{Public} \\ You can either connect directly or use an \textbf{IXP}. An IXP operates on L2 and it forwards the data you sent for a destination AS, by correctly routing the data to it. It acts as really big switch between you an the other AS to enable the direct communication. \textit{Do not mix it with a transit AS as it is not one!}.
\end{enumerate}
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