Issues in Designing the Next Generation Transport Protocol
Dheeraj Sanghi
In Proceedings of Int'l Conf. on Information Technology, Bhubaneshwar, India,
December, 1999
Review by - Rachna Vargiya
This paper discusses the need for the next generation transport layer. According to the author, the existing
transport protocols are unable to provide the requisite quality of service. They are also reaching the limits
of their operation.
For Example, TCP has a window size of only 16 bits, the port number is assigned only 16 bits. TCP’s
congestion control mechanism assumes that a loss of packet indicates congestion in the network. A
temporary bulk loss will result in poor performance for a significant period of time. TCP provides
reliability at excessive cost and UDP is too unreliable. The 3-way handshake introduces a time lag of one
round trip time before data can be sent. IPV6 also requires some changes in the existing protocols.
Modification to existing protocols, like TCP make it very complicated, and therefore open to bugs and
security holes. Also, different applications desire a level of reliability between the two extremes that is
provided by the existing protocols, TCP and UDP.
Their protocol, called TPng, is proposed to support a variety of reliability options. It is proposed to support
both connection less and connection oriented services. It will not support out of band data in TPng. Only
positive acknowledgements are allowed, since negative acknowledgements can at best be used to speed up
transmission, but cannot be relied upon to indicate the arrival of the packet at the other end. They use
cumulative acknowledgement by default, but selective ack will be supported as optional header. There will
be a bit in the header to permit no acknowledgement. TPng will support jumbograms. It should be possible
for an application to suggest whether checksum should be used or not on a per packet basis. There will be a
field in the header to indicate how many data bytes are covered by the checksum. The flow control
mechanism is same as before, that is the sliding window based flow control. TPng will support both byte
streams and records.
The fields in the header are almost similar except a few changes. First field is for the version occupying 4
bits. The packet size in their protocol is a field of 28 bits supporting packets of size 256 MB. The header
size has been increased to a 16 bit field. Both source port and destination port will be of 32 bits. A sequence
number (and acknowledgement sequence number as well) of 64 bits is suggested. There will be a 16 bit
field for sending the computed value of checksum. CRC 16 algorithm is to be used. Checksum length
suggests how much of the packet is covered by the checksum. Checksum flags are used to distinguish
between the different possible cases. There will be 16 service bits which will control the operation of the
protocol or meaning of some of the fields.
The paper puts forth a solution to a problem that most researchers are trying to fight, that is the explosion in
the users of internet and usage of protocols for the same. Existing protocols clearly will not be able to
support all the users for very long. The author says that extension of the current protocol makes it very
complicated. In my opinion, the new idea is also not very different from the current protocol, TCP, and is
pretty much an extension of the same. Only the sizes of certain fields have been increased.The new
protocol, TPng also seems to be dependent upon the IPV6 since it must be able to support jumbograms.
That violates the rule that two layers in a network must be independent of each other.
However, the paper does propose certain novel ideas for reliability and acknowledgements. Overall, the
paper is well written and certainly has certain points worth considering.