Elements of Transport Protocols. The transport service is implemented by a transport protocol used between the two transport entities Transport protocols resemble the data link protocols Both have to deal with error control, sequencing, and flow control, among other issues.
this scheme has a basic flaw: it requires each transport entity to maintain a certain amount of history information indefinitely. If a machine crashes and loses its memory, it will no longer know which connection identifiers have already been used.
Packet lifetime can be restricted to a known maximum using one of the following techniques:
Three protocol scenarios for establishing a connection using a three-way handshake. CR denotes CONNECTION REQUEST. (a) Normal operation, (b) Old CONNECTION REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST and duplicate ACK.
Are of 2 types :
Like telephone s/m: when one party hangs up, the connection is broken.
It treats the connection as 2 separate unidirectional connections and requires each one to be released separately.
Abrupt disconnection with loss of data.
6-14, a, b
(a) Chained fixed-size buffers. (b) Chained variable-sized buffers. (c) One large circular buffer per connection.
(TCP and UDP)
Trivial File Transfer Protocol
Lookup info about a user
World Wide Web
Remote e-mail access
(b) The 2048 bytes of data delivered to the application in a single READ CALL.
This is mainly because ATM is primarily used for transmitting voice and video streams, in which rapid delivery is more important than accurate delivery.
ATM layer outputs 53-byte cells one after another.
It has no error control, no flow control and no other control.
To bridge this gap , ITU defined an end- to-end layer on top of the ATM layer.
This layer is called AAL(ATM Adaptation Layer).
The goal of AAL is
to provide useful services to application programs and
to shield them from the mechanics of chopping data up into cells at the src and reassembling them at the desn.
Real-time service VS Non real-time services.
Constant bit rate services VS variable bit rate services.
Connection-oriented service VS connection less service.
ITU felt only 4 of these were of any use and named them class A,B,C and D.
To handle these 4 classes of service, ITU defined 4 protocols, AAL1 thru AAL4 respectively.
Technical requirements for classes C & D were similar
So combined AAL3 & AAL4 to form AAL3/4
Convergence sublayer (service specific part)
Convergence sublayer (common part)
Segmentation Reassembly sublayer
The upper part of AAL is called Convergence Sublayer.
Its job is to provide interface to the application.
It consists of 2 subparts that is
common to all applications and
an application specific sub part
The functions of each of these parts are protocol dependent but can include msg framing and error detection.
It is also responsible for accepting bit streams and breaking them up into 44-48 bytes for transmition .
Message boundaries are preserved when present
It can add headers and trailers to the data units given to it by the CS to form cell payloads.
These payloads are then given to the ATM layer for transmition.
At the destn the SAR sublayer reassembles the cells into msgs.
The SAR sublayer is basically concerned with cells, but the CS sublayer is concerned with msgs.
It has some additional functions for some service classes
It sometime handles error detection & multiplexing
Is the protocol used for transmiting class A traffic, that is
constant bit-rate ,
connection-oriented traffic- eg: uncompressed audio and video.
Bits are fed in by the application at a constant rate and must be delivered at destn at the same constant rate , with a min. of delay and overhead.
The input is a stream of bits, with no msg boundaries.
For this traffic, error-detecting protocols such as stop-and-wait are not used because the delays introduced by timeouts and retransmition are unacceptable.
Missing cells are reported to the application
The Convergence Sublayer (CS)
detects lost and misinserted cells.
smoothes out incoming traffic to provide delivery of cells at a constant rate.
breaks up the inputp msg or stream into 46 or 47 byte units that r given for SAR for txn.
At the other end it extracts these and reconstructs the original i/p.
The AAL1 CS does not have any protocol headers of its own.
But the AAL1 SAR sublayer does have.
It has 1 byte headercontaining a
3 bit cell seq number SN
to detect missing or misinserted cells
3 bit Sequence Number Protection SNP (like check sum)
Allows correction of single errors & detection of double errors in seq no. field
One bit for parity –even bit parity
P cells used when message boundaries must be preserved
Pointer field -1 byte
Used to give offset of start of next message
Higher order bit is reserved for future use
For pure uncompressed audio/video , or any other data stream in which having a few garbled bits once in a while is not a problem-AAL 1 is adequate.
For compressed audio or video, the rate can vary strongly in time.
Many compression schemes transmit a full video stream periodically, and then send only the differences betwn subsequent frames and the last full frame for several frames.
When the camera is stationary and nothing is moving , the differenz betwn frames are small.
But when the camera is panning rapidly, they r large.
Also msg boundaries must be preserved so that the start of the next full frame can be recognized , even in the presence of lost cells or bad data.
CPI-Common Part Indicator, gives the msg type and the counting unit for the BA size and Length fields.
Btag and Etag are used to frame msgs.
These 2 bytes must be same & incremented by 1 on every new msg sent
The BA size is used for buffer allocation.
Tells receiver how much buffer space to be allocated for message in advance of its arrival
The Length field gives the payload length.
In msg mode length must be equal to BA size..
Trailer has 1 unused byte
After the CS has constructed and added a header and trailer to the msg, it passes the msg to the SAR sublayer, which chops the msg up into 44-byte chunks.
The SAR sublayer inserts 44-byte chunk into the payload of a cell whose format is shown below.
11 single cell msg
00 –middle (continuation of message COM)
01 –end of message (EOM)
10 –beginning of message (BOM)
11 –single segment message (SSM)
SN (Sequence Number) for detecting missing and misinserted cells.
The MID (Multiplexing Identification) is used to keep track of which cell belongs to which session.
Trailer consist of
LI (Lenth Indicator)- indicates payload length
CRC –cell checksum
The AAL 1 thru AAL 3/4 protocols were largely designed by the Tele Communications industry and standardized by ITU without a lot of i/p from the computer industry.
For computer industry a new protocol is invented and it was called SEAL (Simple Efficient Adaptation Layer).
Later it is renamed as AAL 5.
AAL 5 offers both reliable and unreliable services.
It supports both unicast and multicast
For multicast guaranteed delivery is not provided
In msg mode an application can pass a datagram of length 1-65,535 bytes to the AAL layer and have it delivered to the destn, either on a guaranteed or a best effort basis.
Upon arrival in the CS ,a msg is padded out and a trailer added.
The amount of padding( 0-47 bytes) is chosen to make the entire msg be a multiple of 48 bytes.
AAL 5 does not have a CS header , just an 8 byte trailer.
Payload (1-65,535 bytes)
AAL 5 Convergence Sublayer format
UU- User to User
field is not used by the AAL itself.
It is available for a higher layer for its own purpose , like sequencing or muxing.
Length- tells how long the true payload is-in bytes (without padding).
Value =0 used to abort current message in midstream
CRC –is a std 32-bit checksum
The msg is txed by passing it to the SAR sublayer, which does not add any headers or trailers.
Instead it breaks the msg into 48-byte units and passes each of these to the ATM layer for txn.
The main advtg of AAL 5 over AAL 3/4 is the much greater efficiency.
The lack of the seq. number is compensated for by the longer checksum, which an detect lost, misinserted , or missing cells without using seq. numbers.
Within the Internet Community , it is expected that normal way of interfacing to ATM n/ws will be to transport IP packets with the AAL 5 payload field.