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Some characters are used as markers to delineate the frame boundaries. Many `character stuffing` techniques use the `DLE`, `STX` and `ETX` characters of the ASCII character set. `DLE STX` (resp. `DLE ETX`) is used to mark the beginning (end) of a frame. When transmitting a frame, the sender adds a `DLE` character after each transmitted `DLE` character. This ensures that none of the markers can appear inside the transmitted frame. The receiver detects the frame boundaries and removes the second `DLE` when it receives two consecutive `DLE` characters. For example, to transmit frame `1 2 3 DLE STX 4`, a sender will first send `DLE STX` as a marker, followed by `1 2 3 DLE`. Then, the sender transmits an additional `DLE` character followed by `STX 4` and the `DLE ETX` marker.
**1** **2** **3** **4**
`DLE STX` **1** **2** **3** **4** `DLE ETX`
**1** **2** **3** **DLE** **STX** **4**
`DLE STX` **1** **2** **3** **DLE** `DLE` **STX** `4` `DLE ETX`
**DLE STX DLE ETX**
`DLE STX` **DLE** `DLE` **STX** **DLE** `DLE` ETX** `DLE ETX`
`Character stuffing`, like bit stuffing, increases the length of the transmitted frames. For `character stuffing`, the worst frame is a frame containing many `DLE` characters. When transmission errors occur, the receiver may incorrectly decode one or two frames (e.g. if the errors occur in the markers). However, it will be able to resynchronize itself with the next correctly received markers.
Bit stuffing and character stuffing allow recovering frames from a stream of bits or bytes. This framing mechanism provides a richer service than the physical layer. Through the framing service, one can send and receive complete frames. This framing service can also be represented by using the `DATA.request` and `DATA.indication` primitives. This is illustrated in the figure below, assuming hypothetical frames containing four useful bits and one bit of framing for graphical reasons.
We can now build upon the framing mechanism to allow the hosts to exchange frames containing an integer number of bits or bytes. Once the framing problem has been solved, we can focus on designing a technique that allows reliably exchanging frames.
Recovering from transmission errors
In this section, we develop a reliable datalink protocol running above the physical layer service. To design this protocol, we first assume that the physical layer provides a perfect service. We will then develop solutions to recover from the transmission errors.
The datalink layer is designed to send and receive frames on behalf of a user. We model these interactions by using the `DATA.req` and `DATA.ind` primitives. However, to simplify the presentation and to avoid confusion between a `DATA.req` primitive issued by the user of the datalink layer entity, and a `DATA.req` issued by the datalink layer entity itself, we will use the following terminology :
the interactions between the user and the datalink layer entity are represented by using the classical `DATA.req` and the `DATA.ind` primitives
the interactions between the datalink layer entity and the framing sub-layer are represented by using `send` instead of `DATA.req` and `recvd` instead of `DATA.ind`
When running on top of a perfect framing sub-layer, a datalink entity can simply issue a `send(SDU)` upon arrival of a `DATA.req(SDU)` [#fsdu]_. Similarly, the receiver issues a `DATA.ind(SDU)` upon receipt of a `recvd(SDU)`. Such a simple protocol is sufficient when a single SDU is sent. This is illustrated in the figure below.
Unfortunately, this is not always sufficient to ensure a reliable delivery of the SDUs. Consider the case where a client sends tens of SDUs to a server. If the server is faster than the client, it will be able to receive and process all the frames sent by the client and deliver their content to its user. However, if the server is slower than the client, problems may arise. The datalink entity contains buffers to store SDUs that have been received as a `Data.request` but have not yet been sent. If the application is faster than the physical link, the buffer may become full. At this point, the operating system suspends the application to let the datalink entity empty its transmission queue. The datalink entity also uses a buffer to store the received frames that have not yet been processed by the application. If the application is slow to process the data, this buffer may overflow and the datalink entity will not able to accept any additional frame. The buffers of the datalink entity have a limited size and if they overflow, the arriving frames will be discarded, even if they are correct.
To solve this problem, a reliable protocol must include a feedback mechanism that allows the receiver to inform the sender that it has processed a frame and that another one can be sent. This feedback is required even though there are no transmission errors. To include such a feedback, our reliable protocol must process two types of frames :
data frames carrying a SDU
control frames carrying an acknowledgment indicating that the previous frames was correctly processed
These two types of frames can be distinguished by dividing the frame in two parts :
the `header` that contains one bit set to `0` in data frames and set to `1` in control frames
the payload that contains the SDU supplied by the application
The datalink entity can then be modeled as a finite state machine, containing two states for the receiver and two states for the sender. The figure below provides a graphical representation of this state machine with the sender above and the receiver below.
The above FSM shows that the sender has to wait for an acknowledgment from the receiver before being able to transmit the next SDU. The figure below illustrates the exchange of a few frames between two hosts.
Services and protocols
An important aspect to understand before studying computer networks is the difference between a *service* and a *protocol*. For this, it is useful to start with real world examples. The traditional Post provides a service where a postman delivers letters to recipients. The Post precisely defines which types of letters (size, weight, etc) can be delivered by using the Standard Mail service. Furthermore, the format of the envelope is specified (position of the sender and recipient addresses, position of the stamp). Someone who wants to send a letter must either place the letter at a Post Office or inside one of the dedicated mailboxes. The letter will then be collected and delivered to its final recipient. Note that for the regular service the Post usually does not guarantee the delivery of each particular letter. Some letters may be lost, and some letters are delivered to the wrong mailbox. If a letter is important, then the sender can use the registered service to ensure that the letter will be delivered to its recipient. Some Post services also provide an acknowledged service or an express mail service that is faster than the regular service.
Reliable data transfer on top of an imperfect link
The datalink layer must deal with the transmission errors. In practice, we mainly have to deal with two types of errors in the datalink layer :
Frames can be corrupted by transmission errors
Frames can be lost or unexpected frames can appear

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cnp3-ebook / principles/reliability — English

When running on top of a perfect framing sub-layer, a datalink entity can simply issue a `send(SDU)` upon arrival of a `DATA.req(SDU)` [#fsdu]_. Similarly, the receiver issues a `DATA.ind(SDU)` upon receipt of a `recvd(SDU)`. Such a simple protocol is sufficient when a single SDU is sent. This is illustrated in the figure below.
When running on top of a perfect framing sub-layer, a datalink entity can simply issue a `send(SDU)` upon arrival of a `DATA.req(SDU)` [#fsdu]_. Similarly, the receiver issues a `DATA.ind(SDU)` upon receipt of a `recvd(SDU)`. Such a simple protocol is sufficient when a single SDU is sent. This is illustrated in the figure below.
a year ago
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../../principles/reliability.rst:323
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a year ago
Source string age
a year ago
Translation file
locale/pot/principles/reliability.pot, string 93