Network Working Group Andrew G. Malis
Request for Comments: 979 BBN Communications Corp.
March 1986 PSN END-TO-END FUNCTIONAL SPECIFICATION
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.
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
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".
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
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
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
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,
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
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.
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
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
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
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
functionality: they allow the hosts to submit and receive
messages, and they both provide a reliable and sequenced
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
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
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
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).
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.
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.
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
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.
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
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
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