Network Working Group J. McQuillan
RFC #568 BBN-NET
NIC #18971 18 September 1973
Response to RFC 567 -- Cross-Country Network Bandwidth
This note serves as a brief correction to several fundamental errors in
RFC 567 by L. Peter Deutsch.
1. Not all packets are 1000 bits long. This is basic to the network
design.
2. RFNMs are 152 bits long (72 bits of hardware framing and 80 bits of
software identification and addressing). Host Host protocol messages
such as single-characters and allocates are 216 bits long (40 bits
of Host protocol, 8 bits for the character or ALL, and an additional
16 bits of IMP software header). This totals to 736 bits in each
direction, not 4000.
3. The number of single-character messages that can be supported is
therefore over 200 per second, not 37.5 per second. Not only is
such a traffic pattern unlikely, but it can be supported in the IMP
subnetwork much more readily than in most Hosts.
4. Furthermore, if the demand for remote echoing ever exceeds network
capacity, the TIPs and Hosts can simply buffer 2 characters per
message, doubling the effective bandwidth of the network. Of
course, dozens of characters can be packed into a single message
with nearly proportional increases in effective bandwidth, given the
size of the overhead. This buffering happens automatically and
incrementally with increasing load as the natural consequence of
slowed responses.
5. It is most likely that the poor echoing response cited by Deutsch is
not caused by peak network loads. If echoing was coming in 5-
character bursts, there would have to be _1000_ characters per
second coming from users of remote-echo systems to use all the
capacity of 3 50-kilobit paths.
6. This reasoning points up the more serious error in RFC 567: the
problems associated with bad echo response are delay problems, not
bandwidth. In designing the IMP software, we have used a bimodal
model of traffic, and attempted to provide low delay for interactive
RFC 568
traffic, and high throughput rates for bulk data transfers. It is
pointless to try for high data rates with short messages - the
overhead in bits, and also in IMP and Host processor wake-ups, is
too high. The primary factor in echoing performance is delay. As
an extreme example, echoing over a megabit per second satellite link
will lag a second or more behind input, with no bandwidth
limitations at all.
7. We agree that changes to TELNET protocol may well improve
performance by reducing network traffic, and, more importantly,
reducing demands for Host processing. In cases of network paths
with long delay, especially satellite links, such changes are
essential for interactive echoing.
JMcQ/jm
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