Exit Zen
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../../principles/dv.rst:97
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`D` sends its distance vector :math:`[D=0,B=\infty,A=1,C=2,E=1]` to `A` and `E`. `A` recovers routes towards `C` and `E` via `D`.
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a `payload` that contains the user data
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To understand this problem better, let us consider the scenario shown in the figure below, where a server (`A`) attached to a `10 Mbps` link needs to reliably transfer segments to another computer (`C`) through a path that contains a `2 Mbps` link.
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To solve this problem, transport protocols rely on a special timer : the `persistence timer`. This timer is started by the sender whenever it receives an acknowledgment advertising a receive window set to `0`. When the timer expires, the sender retransmits an old segment in order to force the receiver to send a new acknowledgment, and hence send the current receive window size.
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RIP
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`B` sends its distance vector :math:`[B=0,A=\infty,C=1,D=2,E=1]` to `E` and `C`. `C` learns that there is no route anymore to reach `A` via `B`.
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Some headers also include a `length` field, which indicates the total length of the frame or the length of the payload.
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To conclude our description of the basic mechanisms found in transport protocols, we still need to discuss the impact of segments arriving in the wrong order. If two consecutive segments are reordered, the receiver relies on their sequence numbers to reorder them in its receive buffer. Unfortunately, as transport protocols reuse the same sequence number for different segments, if a segment is delayed for a prolonged period of time, it might still be accepted by the receiver. This is illustrated in the figure below where segment `D(1,b)` is delayed.
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Routing Information Protocol. An intradomain routing protocol based on distance vectors that is sometimes used in enterprise networks. RIP is defined in :rfc:`2453`.
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`E` sends its distance vector :math:`[E=0,A=2,C=1,D=1,B=1]` to `D`, `B` and `C`. `D` learns a route towards `B`. `C` and `B` learn a route towards `A`.
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The simplest error detection scheme is the checksum. A checksum is basically an arithmetic sum of all the bytes that a frame is composed of. There are different types of checksums. For example, an eight bit checksum can be computed as the arithmetic sum of all the bytes of (both the header and trailer of) the frame. The checksum is computed by the sender before sending the frame and the receiver verifies the checksum upon frame reception. The receiver discards frames received with an invalid checksum. Checksums can be easily implemented in software, but their error detection capabilities are limited. Cyclical Redundancy Checks (CRC) have better error detection capabilities [SGP98]_, but require more CPU when implemented in software.
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In this network, the segments sent by the server reach router `R1`. `R1` forwards the segments towards router `R2`. Router `R1` can potentially receive segments at `10 Mbps`, but it can only forward them at `2 Mbps` to router `R2` and then to host `C`. Router `R1` includes buffers that allow it to store the packets that cannot immediately be forwarded to their destination. To understand the operation of a reliable transport protocol in this environment, let us consider a simplified model of this network where host `A` is attached to a `10 Mbps` link to a queue that represents the buffers of router `R1`. This queue is emptied at a rate of `2 Mbps`.
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Ambiguities caused by excessive delays
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RIR
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Self clocking
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../../principles/transport.rst:793
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To deal with this problem, transport protocols combine two solutions. First, they use 32 bits or more to encode the sequence number in the segment header. This increases the overhead, but also increases the delay between the transmission of two different segments having the same sequence number. Second, transport protocols require the network layer to enforce a `Maximum Segment Lifetime (MSL)`. The network layer must ensure that no packet remains in the network for more than MSL seconds. In the Internet the MSL is assumed [#fmsl]_ to be 2 minutes :rfc:`793`. Note that this limits the maximum bandwidth of a transport protocol. If it uses `n` bits to encode its sequence numbers, then it cannot send more than :math:`2^n` segments every MSL seconds.
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Regional Internet Registry. An organization that manages IP addresses and AS numbers on behalf of :term:`IANA`.
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Connection release
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root nameserver
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Consider now that the link between `D` and `E` fails. The network is now partitioned into two disjoint parts: (`A` , `D`) and (`B`, `E`, `C`). The routes towards `B`, `C` and `E` expire first on router `D`. At this time, router `D` updates its routing table.
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