note on recent arpanet performance problem


Doug Hunt (hunt@bbnccj.ARPA)
Tue, 8 Apr 86 15:14:44 EST


        Status of Remedial Action for Arpanet Performance Problems

This note describes an Arpanet performance problem, together with the
steps being taken to correct it. The discussion covers (1) the
symptoms, (2) an explanation of the problem, (3) the recommended
solution, and (4) a timetable for implementing the solution. This note
concludes with a recommended procedure for reporting Arpanet performance
problems to the Arpanet Manager at the DDN Program Management Office.

The Symptoms

Basically, the Arpanet appears sluggish -- interactive as well as bulk
transfer traffic can be subjected to very large delays. This can
manifest itself as an overflowing typeahead (input) buffer to a TAC, or
a file transfer that takes much longer than should be required for the
file size and data path bandwidths involved.

The Problem

The BBNCC Network Analysis Group considered a number of hypotheses that
could explain the sluggish network behavior. They have found that much
of the performance degradation in the Arpanet can be traced to host
blocking owing to a lack of per-connection resources in PSNs. This
blocking would exhibit the symptoms described above.

A host must have a PSN allocate to it certain resources in order to
establish and use a host-host network connection. To establish the
connection, both the source and destination hosts must be allocated
"connection blocks" by their respective source and destination PSNs.
Having done so, to send a message a host must be allocated a "message
number". The connection blocks and message numbers are resources
internal to the PSN; until a PSN allocates these resources to a host,
the host is blocked.

Analysis of the Arpanet in recent months shows that, for many PSNs,
there is excessive blocking of attached hosts because of a shortage of
connection blocks. Twelve PSNs in particular exhibit this problem
consistently. These PSNs are: SRI2, RCC5, STAN11, DCEC20, ISI22, ISI27,
PURDU37, SRI51, LBL68, SAC80, BBN82, and WISC94. Alleviating the
connection block shortage problem for these and other Arpanet PSNs
should provide a significant improvement in network performance as seen
by hosts.

The Solution

The PSN release used on the Arpanet, "Release 3/4", supports only 73
connection blocks in each direction -- enough to support 73 full-duplex
streams. Measurements have shown that, for many PSNs including those
listed above, there is excessive contention for these 73 connection
blocks. Analysis of the traffic patterns of the attached hosts
indicates that increasing the number of connection blocks to 255 -- the
number supported in PSN Release 5 -- should greatly reduce the
likelihood of hosts blocking owing to a connection block shortage.

The Timetable

On March 12, the DDN Program Management Office (PMO) issued a "Network
Change Directive", authorizing the installation of PSN release 5 to most
Arpanet sites prior to May 2, 1986. A few sites will not receive the
new release until later, since those sites have not yet had the hardware
upgrade necessary to support PSN release 5. Those sites with the most
serious performance problems will be the first sites to receive the new
release. PSN release 5 has already been subjected to extensive
operational use, as the DDN PMO has previously authorized the
installation of PSN release 5 on other networks, such as the MILNET.

Recommended Procedure for Reporting Arpanet Performance Problems

In order to facilitate resolution of performance problems on the Arpanet
it is important that the Arpanet Manager at the DDN PMO be informed of
problems noticed by Arpanet users. Currently, the Arpanet Manager at
the DDN PMO is Captain Callaway. Arpanet users are encouraged to
include this office, with mailbox "arpanetmgr 'at' ddn1", on network
mail regarding Arpanet problems. The description of the problem should
be as detailed as possible. Include information such as:

1. description of problem
2. time of day problem occurs
3. duration of problem
4. host you are using
5. gateway you are using (if applicable)
6. host you are attempting to reach
7. general description of computing you are engaged in -- e.g., "I am
    transferring files from my host to host XYZ. Approximate size of
    file is 64k bytes".
8. provide any other information you think would be helpful in
    resolving the problem.

Since the DDN PMO establishes the policies for operating and managing
the Arpanet, it is important that the PMO Arpanet Manager be aware of
problems experienced by Arpanet users.



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