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PSN End-to-End functional specification
RFC 979

Document Type RFC - Unknown (March 1986)
Last updated 2013-03-02
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RFC 979
Network Working Group                                    Andrew G. Malis
Request for Comments: 979                       BBN Communications Corp.
                                                              March 1986


Status of this Memo

   This memo is an updated version of BBN Report 5775, "End-to-End
   Functional Specification".  It has been updated to reflect changes
   since that report was written, and is being distributed in this form
   to provide information to the ARPA-Internet community about this
   work.  The changes described in this memo will affect AHIP (1822
   LH/DH/HDH) and X.25 hosts directly connected to BBNCC PSNs.
   Information concerning the schedule for deployment of this version of
   the PSN software (Release 7.0) in the ARPANET and the MILNET can be
   obtained from DCA.  Distribution of this memo is unlimited.

1  Introduction

   This memo contains the functional specification for the new BBNCC PSN
   End-to-End (EE) protocol and module (PSN stands for Packet Switch
   node, and has previously been known as the IMP).  The EE module is
   that portion of the PSN code which is responsible for maintaining EE
   connections that reliably deliver data across the network, and for
   handling the packet level (level 3) interactions with the hosts.  The
   EE protocol is the peer protocol used between EE modules to create,
   maintain, and close connections. The new EE is being developed in
   order to correct a number of deficiencies in the old EE, to improve
   its performance and overall throughput, and to better equip the PSN
   to support its current and anticipated host population.

   The initial version of the new EE is being fielded in PSN Release
   7.0.  Both the old and new EEs are resident in the PSN code, and each
   PSN may run either the old or the new EE (but not both) at any time,
   under the control of the Network Operations Center (NOC).  The NOC
   has facilities for switching individual PSNs or the entire network
   between the old and new EEs.  When the old EE is running, PSN 7.0's
   functionality is equivalent to that provided by PSN 6.0, and the
   differences listed in this memo do not apply.  Hosts on PSNs running
   the old EE cannot interoperate with hosts on PSNs running the new EE.

   There are two additional sections following this introduction.
   Section two describes the motivation and goals driving the new EE

   Section three contains the new EE's functional specification.  It
   describes the services provided to the various types of hosts that

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PSN End-to-End Functional Specification

   are supported by the PSN, the addressing capabilities that it makes
   available, the functionality required for the peer protocol, and the
   performance goals for the new EE.

   Two notes concerning terminology are required.  Throughout this
   document, the units of information sent from one host to another are
   referred to as "messages", and the units into which these messages
   are fragmented for transmission through the subnetwork are referred
   to as "subnet packets" or just "packets".  This differs from X.25's
   terminology; X.25 "packets" are actually messages.  Also, in this
   report the term "AHIP" is used to refer to the ARPANET Host-IMP
   Protocol described in BBN Report 1822, "Specifications for the
   Interconnection of a Host and an IMP".

2  Motivation

   The old EE was developed almost a decade ago, in the early days of
   packet-switching technology.  This part of the PSN has remained
   stable for eight years, while the environment within which the
   technology operates has changed dramatically.  At the time the old EE
   was developed, it was used in only one network, the ARPANET.  There
   are now many PSN-based networks, some of which are grouped into
   internets.  Originally, AHIP was the only host interface protocol,
   with NCP above it.  The use of X.25 is now rapidly increasing, and
   TCP/IP has replaced NCP.

   This section describes the needs for more flexibility and increases
   in some of the limits of the old EE, and lists the goals which this
   new design should meet.

   2.1  Benefits of a New EE

      Network growth and the changing network environment make improved
      performance, in terms of increasing the PSN's throughput, an
      important goal for the new EE.  The new EE reduces protocol
      traffic overhead, thereby making more efficient use of network
      line bandwidth and transit PSN processing power.

      The new EE provides a set of network transport services which are
      appropriate for both the AHIP and X.25 host interfaces, unlike the
      old EE, which is highly optimized for and tightly tied to the AHIP
      host interface.

      The new EE has an adjustable window facility instead of the old
      EE's fixed window of eight outstanding messages between any host
      pair.  The old EE applies this limit to all traffic between a pair
      of hosts; it has no notion of multiple independent channels or

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RFC 979                                                       March 1986
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      connections between two hosts, which the new EE allows.  A network
      with satellite trunking, and consequently long delays, is an
      example of where the new window facility increases the EE
      throughput that can be attained.  TACs and gateways provide
      another example where the old EE's fixed window limits throughput;
      all of the traffic between a host and a TAC or a gateway currently
      uses the same EE connection and is subject to the limit of eight
      outstanding messages, even if more than one user's traffic flows
      are involved.  With the new EE, this restriction no longer

      Supportability also motivates rewriting the EE software.  The new
      EE can be written using more modern techniques of programming
      practice, such as layering and modularity, which were not as well
      understood when the old EE was first designed, and which will make
      the EE easier to support and to enhance.

      Finally, the new EE includes a number of new features that improve
      the PSN's ability to provide services which are more closely
      optimized to what our customers need for their applications.
      These include new addressing capabilities, precedence levels,
      end-to-end data integrity checks, and monitoring and control

   2.2  Goals for the New EE

      The new EE's X.25 support is greatly improved over that provided
      by the old EE.  One element of this improvement is at least
      halving the amount of per-message EE protocol overhead.  Another
      element is the unification of the different storage allocation
      mechanisms used by the old EE and X.25 modules, where data
      transferred between the old EE and X.25 must be copied from one
      type of structure to the other.

      The new EE presents, as much as possible, a non-blocking interface
      to the hosts.  If a host overwhelms the PSN with traffic, the PSN
      ultimately has to block it, but this should happen less frequently
      than at present.

      In the old EE, all of the hosts contend for the same pool of
      resources.  In the new EE, fairness is enforced in resource
      allocation among different hosts through per-host minimum
      allocations for buffers and connection blocks as part of a general
      buffer management system.  This insures that no host can be
      completely "shut out" of service by the actions of another host at
      its PSN.

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      The EE supports four precedence levels and optional (on a per-
      network basis) preemption features.

      Addressing capabilities have been extended to include hunt groups.

      Instead of a fixed window of eight outstanding messages between
      any host pair, the maximum window size on an EE connection is
      configurable to a maximum of 127.  The EE allows host pairs to set
      up multiple connections, each with an independent window.

      A result of the old EE's reliance on destination buffer
      reservation is that subnet packets can be lost if an intermediate
      node goes down.  The new EE uses source buffering with
      retransmission in order to provide more reliable service.

      The new EE has a duplex peer protocol, allowing acknowledgments to
      be piggybacked on reverse traffic to reduce protocol overhead.
      When reverse traffic is not available, acknowledgments are
      aggregated and sent together.

      The result of this development will be end-to-end software with
      greater performance, supportability, and functionality.

3  End-to-End Functionality

   This section contains the new EE's functional specification.  It
   describes the services provided to the various types of hosts that
   are supported by the new EE, the addressing capabilities that it
   makes available, the functionality required for the peer protocol,
   the performance goals for the new EE, the EE's network management
   specification, and provisions for testing and debugging.

   3.1  Network Layer Services

      The most important part of designing any new system is determining
      its external functionality.  In the case of the new EE, this is
      the network layer services and interfaces presented to the hosts.

      3.1.1  Common Functionality

         The following three sections list details concerning the new
         EE's support for the X.25, AHIP and Interoperable network layer
         services.  In the interest of brevity, however, additional
         functionality available to all three services is listed herein:

            o  In order to check data integrity as packets cross through
               the network, the old EE relies on a trunk-level,

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               hardware/ firmware-generated, per-packet CRC code (which
               is either 16 or 24 bits in size, depending on the PSN-PSN
               trunk protocol in use) and a software-generated
               per-packet 16-bit checksum.  Neither of these are
               end-to-end checks, only PSN-to-PSN checks.  For the new
               EE, the software checksum has been extended to be an
               optional 32-bit end-to-end checksum, and the per-packet
               software checksum has been reduced to a parity bit.

               The network administration now has a choice as to which
               is most important, efficient utilization of network
               trunks (due to the reduced size of the per-packet
               headers), or strong checks on data integrity.

               Those hosts that require strong data integrity checking
               can request, in their configuration, that all messages
               originating from this host include a 32-bit per-message
               end-to-end checksum.  This checksum is computed in the
               source PSN, is ignored by tandem PSNs along the path, and
               is checked in the destination PSN.  If the checksum does
               not check, the EE's regular source retransmission
               facilities are used to have the message resent.

            o  The old EE's access control mechanism allows 15 separate
               communities of interest to be defined, and uses an
               unnecessarily complicated algorithm to define which
               communities can intercommunicate.  This mechanism is
               being expanded to allow 32 communities of interest,
               rather than the previous limit of 15.  The feature that
               allowed hosts to communicate with a community without
               actually being a member of that community has been
               removed because it was never utilized.

            o  The addressing capabilities of the PSN have been improved
               by the new EE.  In addition to continuing to support the
               old EE's logical addressing facility, hunt groups (for
               both AHIP and X.25 hosts) have been added.  These are
               described further in Section 3.2.

            o  Connection  block  preemption  is  supported on a
               configurable per-network basis.  If a network is
               configured to use  connection block preemption, then
               lower-precedence connections can be closed by the  PSN,
               if  necessary,  in  order  to  maintain  configured
               reserves of PSN resources for higher-precedence

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            o  The new EE supports congestion control and improved
               resource allocation policies which ensure fairness and
               graceful degradation of service under extreme load.
               Certain resources can be prereserved to each host port,
               and each port can also be limited in its use of shared
               resources.  This ensures that no host can be totally shut
               out from PSN resources by the actions of other hosts at
               the same PSN.  In addition, each PSN is sensitive to
               congestion in both of the PSNs at the endpoints of each
               connection, and it can exert backpressure (flow control)
               on hosts, as necessary, to prevent congestion.

      3.1.2  X.25

         The new EE's X.25 service represents an improvement over the
         X.25 service available from the old EE.  The following
         paragraphs summarize the X.25 support in the new EE:

            o  The new EE provides both DDN Standard and Basic X.25
               service, as described in BBN Reports 5476, "DDN X.25 Host
               Interface Specification," and 5500, "C/30 PSN X.25
               Interface Specification," respectively.  In addition, the
               description of DDN Standard Service, Version 2, is found
               in Section 3.1.4 of this document.

            o  All data packets and call requests are source-buffered in
               the source PSN to provide a better level of reliability
               for network traffic.  This should keep the network from
               issuing a reset on an open connection as a result of a
               lost packet in the subnet or any other occasional
               subnetwork failure.  Except in cases of extreme network
               or node congestion, recovery from lost subnet packets is
               automatic and transparent to the end user or host.

            o  Both local and end-to-end significance for host window
               advancement (based upon the D bit from the host) are
               planned, but only end-to-end significance is included in
               the initial release (the old EE did not include local
               significance).  The D bit is passed through the network

      3.1.3  AHIP

         Another service provided by the new EE is defined in BBN Report
         1822, "Specifications for the Interconnection of a Host and an
         IMP", as amended by Report 5506, "The ARPANET 1822L Host Access
         Protocol".  This ARPANET Host-IMP Protocol (AHIP) service is

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PSN End-to-End Functional Specification

         supported in a backwards-compatible manner by the new EE; since
         this is a BBNCC-private protocol, the new EE can improve the
         service to better match its current uses (the AHIP protocol was
         first designed over twelve years ago).  The main changes to
         AHIP are to remove the absolute eight-message-in-flight
         restriction for connection-based traffic, and to improve the
         PSN's "datagram" support for non-connection-based traffic.

         For this new support, datagram service is planned (for PSN
         Release 8.0) to include fragmentation and reassembly by the
         network, but without requiring the network overhead used by
         connections, and without the reliability, message sequencing,
         and duplicate detection that connections provide.  However,
         "destination dead" indications will be provided to the source
         host where possible and appropriate.

         With the new EE, hosts are also able to create multiple
         connections between host pairs by using the 8-bit "handling
         type" field to specify up to 256 different connections.  The
         field is divided into high-order bits that specify the
         connection's precedence, and low-order bits that distinguish
         between multiple connections at the same precedence level.
         Since the new EE is using four precedence levels, the handling
         type field is used to specify 64 different connections at each
         of the four precedence levels.

         AHIP connections will continue to be implicitly created and
         automatically torn down after a configurable period (nominally
         three minutes) of inactivity, or because of connection block

         To summarize the new end-to-end's AHIP support:

            o  The old EE's AHIP services are supported in a
               backwards-compatible manner (except where listed below).

            o  The old EE's uncontrolled (subtype 3) message service
               will be replaced, in PSN Release 8.0, by the datagram
               service mentioned above.  This service will provide
               fragmentation and reassembly, so that there is no special
               restriction on the size of datagrams; will not insure
               that messages are delivered in order or unduplicated, or
               provide a delivery confirmation; will notify the source
               host if the destination host or PSN is dead; will not
               require the connection block overhead associated with
               connections; and may lose messages in the subnet, without
               notification to the source host, in the event of subnet

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RFC 979                                                       March 1986
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               congestion or component failures.  This service could be
               useful for applications that do not need the absolute
               reliability or sequentiality of connections and therefore
               wish to avoid their associated overhead.

               Datagrams are not supported by the new EE in PSN Release

            o  Connections no longer have the old EE's "eight messages
               in flight" restriction, and a pair of hosts can be
               connected with up to 256 simultaneous implicit
               connections.  In addition, multiple precedence levels are

            o  The new EE supports interoperability between AHIP and
               X.25 hosts (see Section 3.1.4 for further details).

            o  AHIP local, distant, and HDH (both message and packet
               mode) hosts are supported.  The new EE does not support
               VDH hosts.  VHA and 32-bit leaders are supported.

            o  Packet-mode HDH has been extended to allow longer packet
               data frames (see BBN Report 1822, Appendix J, for a
               description of the HDH protocol).  Middle packet frames
               can now contain up to 128 octets of data, rather than the
               previous 126 (although there must still be an even number
               of octets per frame).  Last packet frames can now contain
               up to 127 octets of data, rather than the previous 125,
               and the number of octets need not be even.  However, the
               maximum total message size is still 1007 data octets. The
               PSN uses these new packet frame size limits when sending
               packet frames to packet-mode HDH hosts unless the host is
               configured to allow only 126-octet frames.  In addition,
               there are restrictions on packet-mode HDH when
               interoperating with DDN Standard X.25 hosts; these
               restrictions are discussed in Section 3.1.4.

      3.1.4  Interoperability (DDN Standard X.25)

         One of the main goals of the new EE is to provide
         interoperability between AHIP and X.25 hosts.  On the surface,
         this may appear difficult, since the two host access protocols
         have little in common: X.25 presents a connection-oriented
         interface with explicit windowing, while AHIP presents a
         reliable datagram-oriented interface with implicit flow
         control.  However, they both have the same underlying

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RFC 979                                                       March 1986
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         functionality:  they allow the hosts to submit and receive
         messages, and they both provide a reliable and sequenced
         delivery service.

         The key to interoperability is the fact that in the new EE,
         both X.25 and AHIP connections use the same underlying
         protocols and constructs.  The new EE has AHIP and X.25 Level 3
         modules that translate between the specific host protocols and
         the EE mechanisms.  Since these Level 3 host modules share a
         common interface with the EE, the fact that the two hosts on
         either side of an EE connection are not using the same access
         protocol is largely hidden.

         As a result, the new EE supports basic interoperability.
         However, there are some special cases that need to be mapped
         from one protocol to the other, or just not supported because
         no mapping exists.  For example, AHIP has no analogue of X.25's
         Interrupt packet, while X.25 does not support an unreliable
         datagram service such as AHIP's subtype 3 messages.  For each
         of these cases, the recommendations of BBN Report 5476, "DDN
         X.25 Host Interface Specification," have been followed.

         The interoperable service provided by the new EE is called DDN
         Standard Service, Version 2.  Standard Service, Version 1, is
         defined in BBN Reports 5760, "Preliminary Interoperable
         Software Design," and 5900 Revision 1, "Supplement to BBN
         Report Nos. 5476 and 5760".

         The major differences between Versions 1 and 2 are:

            o  Version 2 offers improved performance over Version 1.

            o  The EE now provides four precedence levels.  Therefore,
               the four precedence levels allowed in the DDN-private
               Call Precedence Negotiation are mapped directly to subnet
               precedence levels, instead of being collapsed into two
               subnet precedence levels as in Version 1.

            o  On an interoperable connection, the X.25 protocol ID in
               an X.25-originated message is translated to an AHIP link
               number (the upper eight bits of the message-ID field)
               using a lookup table.  Version 1 supports only the IP
               protocol ID and corresponding link number of 155
               (decimal).  Version 2 allows new values to be added to
               the lookup table.  At present, IP is the only protocol
               supported.  In addition, the AHIP link number is also
               used to distinguish one connection from another.  This

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               guarantees that when an AHIP host is sending messages to
               an X.25 host, messages using different link numbers come
               into the X.25 host on different X.25 connections.

            o  Since a "translation module" is no longer necessary in
               the PSN, interoperable connections now have end-to-end
               significance, with a direct correspondence between X.25
               RRs and AHIP RFNMs.  This preserves the meaning of the
               RFNM as defined in Report 1822.  Although Release 7.0
               only offers end-to-end significance, the D bit is passed
               transparently on Standard Service connections between two
               X.25 hosts.

            o  Up to 256 simultaneous connections are supported between
               host pairs that are using the same addresses and
               precedence levels.  Version 1 only supported one such

         The following Version 1 services are not offered by Version 2:

            o  Permanent Virtual Circuits.

            o  X.25 protocol bypass (a BBN-private service).

         A number of items in Report 5760 were the subject of some
         discussion, and three of them need to be specifically mentioned
         here.  First, for DDN Standard Service, Version 1,
         acknowledgments have local significance only, and the D bit
         must be set to 0 in the call request.  In DDN Standard Service,
         Version 2, only end-to-end significance is being provided, as
         was mentioned above.  For backwards compatibility with Version
         1, the D bit can be set to 0 or 1 in a call, but hosts are
         advised that only end-to-end significance is provided in
         Version 2.

         Second, non-standard Default Precedence is not supported by
         either Standard Service Version 1 or Version 2.  Support for
         this facility in Version 1 was withdrawn at the request of DCA.

         Third, although DTEs are allowed to request maximum packet
         sizes of 16, 32, and 64 octets, the DCE always negotiates up to
         128 octets, as per Section 6.12 ("Flow Control Parameter
         Negotiation") of the CCITT 1984 X.25 Recommendation.  This is
         true of both Version 1 and Version 2.  Since IP and TCP are
         required when Standard Service is in use, this is a reasonable
         restriction (due to the length of IP and TCP headers).

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         One issue must be raised concerning interoperability between
         X.25 and packet-mode HDH hosts.  In order to efficiently
         interoperate, packet-mode HDH hosts should completely fill
         their middle packet frames with 128 octets of data.
         Packet-mode HDH hosts that send or require receiving middle
         packet frames with less than 128 octets of data can still
         interoperate with X.25 hosts, but at a greater expense of PSN
         CPU resources per message.

   3.2  Addressing

      The old EE supports, for both AHIP and X.25 hosts, two forms of
      host addressing, physical and logical.

      Physical addressing consists of identifying a host port by the
      combination of its PSN number and the port number on that PSN.
      Logical addressing allows an arbitrary 16-bit "name" to refer to a
      list of one or more host ports.  The EE tries to open a connection
      to one of the ports in the list according to the criterion chosen
      for that name: first reachable in the ordered list, closest port
      (in terms of routing delay), or round-robin load sharing.

      For the new EE, logical addressing is supported on an explicit
      per-connection basis: all logical-to-physical address translations
      take place in the source PSN when a connection is established.
      Once this translation has occurred, all data messages on the
      connection are sent to the same physical address.

      In addition, hunt groups are also now supported for both X.25 and
      AHIP hosts.  This new capability allows host ports on a
      destination PSN to be combined into a "hunt group".  The ports
      share the same group identifier, and incoming connections are
      evenly spread over the ports in the group.  This differs from
      logical addressing's load sharing, where all name translations
      take place in the source PSN, the different ports can be on any
      number of PSNs, and the load sharing is on a per-source-PSN basis.
      By contrast, all of the host ports in a hunt group are on the same
      PSN, the group-to-port resolution takes place in the destination
      PSN, and the load sharing of incoming connections can be
      guaranteed over the ports by the destination PSN.  For X.25, hunt
      groups comply with Section 6.24 of the 1984 X.25 Recommendation.
      Note that Called Line Address Modification is not supported.

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   3.3  Protocol Functionality

      The EE peer protocol runs between EE modules in PSNs on either end
      of an EE connection.  This protocol and its mechanisms have to
      perform the following functions:

         o  Provide full duplex connections (the old EE provides simplex
            connections, and any two-way traffic, such as that generated
            by TCP, requires two subnet connections).

         o  Open a connection and optionally send a full message's worth
            of data as a part of the open request (the old EE requires a
            separate opening sequence in each direction before data can

         o  Reliably send connection-oriented messages, properly
            fragmented/reassembled and sequenced.

         o  Close (clear) a connection (normally, or in a "clean-up"
            mode after a host or PSN dies).

         o  Reset a connection (like the X.25 reset procedure).

         o  Be able to send a limited amount of out-of-band traffic
            associated with a connection (like the X.25 interrupt).

         o  Use source buffering with message retransmission (after a
            timeout) to insure delivery (the old EE depends on
            destination buffer preallocation, which adds protocol
            overhead and cannot recover from lost packets in the

         o  Use an internal connection window of up to 127 messages.

         o  Support two types of ACKs, Internal ACKs (IACKs) and
            External ACKs (EACKs), which are further described following
            this list

         o  Have an inactivity timer for each connection.  For AHIP and
            Standard X.25, the connection is closed if the timer fires.
            For Basic X.25, the EE uses an internal Hello/I-Heard-You
            sequence with the PSN on the other end of the connection to
            check if the other end's host or PSN is still alive.  If
            not, then the connection is closed.

         o  Be able to gracefully handle resource shortages and avoid
            reassembly lockup problems.

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      As mentioned above, the protocol supports two types of
      acknowledgments, IACKs and EACKs.  Both types of ACKs apply to
      messages only; individual packets are not acknowledged.  Since
      windowing is being used, an individual ACK can be used to
      acknowledge more than one message.

      IACKs are used to cancel the retransmission timer and free source
      buffering, and are sent when a message has been completely
      reassembled and delivered from the EE to either the AHIP or X.25
      level 3 module.  This allows the EE to avoid unnecessary message
      retransmissions, and speeds up the process of freeing source
      buffering when destination hosts are slow to accept messages or,
      in the case of X.25, slow to advance the PSN's window to the
      destination (X.25 does not specify any time limit for a host to
      acknowledge that it received a message).

      EACKs are used to advance the end-to-end window and to cause one
      or more end-to-end X.25 RRs or AHIP RFNMs to be sent to the source
      host.  An EACK is sent when an X.25 host acknowledges a message or
      when an AHIP host actually receives it.

      Both types of ACKs are piggybacked, if possible, on reverse
      traffic to the source PSN (for any connection).  Whenever a packet
      is sent to another PSN, it is filled to the maximum allowed
      subnetwork packet size with any outstanding ACKs that may be
      waiting to be sent to that PSN.  After a configurable period, all
      outstanding ACKs for the same PSN are aggregated together and
      sent.  In addition, succeeding ACKs for the same connection can be
      combined into one, and EACKs can be used to imply that a message
      is being IACKed as well (if the destination host is speedy enough
      when receiving or acknowledging messages to allow IACKs and EACKs
      to be combined).

      This ACK aggregation timer interacts with the source buffering
      retransmission timer in the following manner:  whenever a message
      is sent from a host on one PSN to a host on a second PSN, an IACK
      is sent back to the first PSN when the message has been completely
      reassembled by the destination EE, and an EACK is sent when it has
      been delivered (and perhaps ACKed) by the destination host.  The
      IACK must make it back to the source PSN within the limits of the
      retransmission timer, or unnecessary retransmissions could be sent
      across the network.  This limits the ACK aggregation timer to
      being shorter than the source buffering retransmission timer.

      If the destination host is quick enough when accepting traffic
      from its PSN (with respect to the ACK aggregation timer), then the
      EACK can be combined with the IACK, and only the EACK would be

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RFC 979                                                       March 1986
PSN End-to-End Functional Specification

      sent.  If the destination host is even quicker, multiple IACKs and
      EACKs could be combined into one EACK.  In the best case, if there
      is a steady stream of traffic going between the two PSNs in both
      directions (but not necessarily over the same connection or even
      between the same pairs of hosts in each direction), then all of
      the IACKs and EACKs could be piggybacked on data packets and cause
      no additional network packets other than the data packets already
      required to send the data messages across the network. In the
      worst case, however, such as when there is only a one-way flow
      from a source PSN to a destination PSN and the destination host is
      very slow to accept the messages from the network, then each data
      message could result in separate IACKs and EACKs being sent back
      to the source PSN in individual packets.  However, even though the
      IACKs may cause additional packets to cross the network, they are
      still less expensive than the source retransmissions that they are
      used to prevent, and they also serve to free up valuable source
      buffering space.

   3.4  Performance and Capacity Goals

      Performance and capacity goals for the new EE include:

         o  Throughput:  The AHIP host-host and host-trunk maximum
            throughput (in packets/second) will be at least as good as
            at present, and should improve for those situations that
            currently entail traffic limitations based upon the old EE's
            underlying protocol.  The current X.25 intrasite host-host
            and host-trunk throughput will each improve by at least 50%.
            The store-and-forward throughput for the new EE's X.25-based
            traffic will improve by at least 100%.

         o  Connections:  The new EE will support at least 500
            simultaneous connections per PSN, and will be able to handle
            at least 50% more call setups per second than at present.

         o  Buffering:  The EE will have at least 400 packet buffers
            available to source-buffer and/or reassemble messages.

         o  Network size:  The EE protocol and module will use data
            structure and message field sizes sufficient to support at
            least up to 255 hosts per PSN and 1023 PSNs per network
            (however, other PSN protocols and modules presently
            constrain these figures to 63 hosts per PSN and 253 PSNs per

         o  Other:  The EE will support four message precedence levels

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RFC 979                                                       March 1986
PSN End-to-End Functional Specification

            and a maximum message length of 1024 bytes.  For logical
            addressing, the EE will support at least 1024 logical names
            and at least 2048 address mappings per network.

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