rfc568.txt

RFC 相关的技术文档

Received at NIC 21-Sept-73


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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