RFC 1122
Requirements for Internet Hosts - Communication Layers
Pages: 116
Internet Standard: 3
→ Errata
STD 3 is also: 1123
Updates: 0793
Updated by: 1349 4379 5884 6093 6298 6633 6864 8029 9293
Internet Standard: 3
→ Errata
STD 3 is also: 1123
Updates: 0793
Updated by: 1349 4379 5884 6093 6298 6633 6864 8029 9293
Part 4 of 5 – Pages 72 to 95
3.5 INTERNET LAYER REQUIREMENTS SUMMARY | | | | |S| | | | | | |H| |F | | | | |O|M|o | | |S| |U|U|o | | |H| |L|S|t | |M|O| |D|T|n | |U|U|M| | |o | |S|L|A|N|N|t | |T|D|Y|O|O|t FEATURE |SECTION | | | |T|T|e -------------------------------------------------|--------|-|-|-|-|-|-- | | | | | | | Implement IP and ICMP |3.1 |x| | | | | Handle remote multihoming in application layer |3.1 |x| | | | | Support local multihoming |3.1 | | |x| | | Meet gateway specs if forward datagrams |3.1 |x| | | | | Configuration switch for embedded gateway |3.1 |x| | | | |1 Config switch default to non-gateway |3.1 |x| | | | |1 Auto-config based on number of interfaces |3.1 | | | | |x|1 Able to log discarded datagrams |3.1 | |x| | | | Record in counter |3.1 | |x| | | | | | | | | | | Silently discard Version != 4 |3.2.1.1 |x| | | | | Verify IP checksum, silently discard bad dgram |3.2.1.2 |x| | | | | Addressing: | | | | | | | Subnet addressing (RFC-950) |3.2.1.3 |x| | | | | Src address must be host's own IP address |3.2.1.3 |x| | | | | Silently discard datagram with bad dest addr |3.2.1.3 |x| | | | | Silently discard datagram with bad src addr |3.2.1.3 |x| | | | | Support reassembly |3.2.1.4 |x| | | | | Retain same Id field in identical datagram |3.2.1.5 | | |x| | | | | | | | | | TOS: | | | | | | | Allow transport layer to set TOS |3.2.1.6 |x| | | | | Pass received TOS up to transport layer |3.2.1.6 | |x| | | | Use RFC-795 link-layer mappings for TOS |3.2.1.6 | | | |x| | TTL: | | | | | | | Send packet with TTL of 0 |3.2.1.7 | | | | |x| Discard received packets with TTL < 2 |3.2.1.7 | | | | |x| Allow transport layer to set TTL |3.2.1.7 |x| | | | | Fixed TTL is configurable |3.2.1.7 |x| | | | | | | | | | | | IP Options: | | | | | | | Allow transport layer to send IP options |3.2.1.8 |x| | | | | Pass all IP options rcvd to higher layer |3.2.1.8 |x| | | | |
IP layer silently ignore unknown options |3.2.1.8 |x| | | | | Security option |3.2.1.8a| | |x| | | Send Stream Identifier option |3.2.1.8b| | | |x| | Silently ignore Stream Identifer option |3.2.1.8b|x| | | | | Record Route option |3.2.1.8d| | |x| | | Timestamp option |3.2.1.8e| | |x| | | Source Route Option: | | | | | | | Originate & terminate Source Route options |3.2.1.8c|x| | | | | Datagram with completed SR passed up to TL |3.2.1.8c|x| | | | | Build correct (non-redundant) return route |3.2.1.8c|x| | | | | Send multiple SR options in one header |3.2.1.8c| | | | |x| | | | | | | | ICMP: | | | | | | | Silently discard ICMP msg with unknown type |3.2.2 |x| | | | | Include more than 8 octets of orig datagram |3.2.2 | | |x| | | Included octets same as received |3.2.2 |x| | | | | Demux ICMP Error to transport protocol |3.2.2 |x| | | | | Send ICMP error message with TOS=0 |3.2.2 | |x| | | | Send ICMP error message for: | | | | | | | - ICMP error msg |3.2.2 | | | | |x| - IP b'cast or IP m'cast |3.2.2 | | | | |x| - Link-layer b'cast |3.2.2 | | | | |x| - Non-initial fragment |3.2.2 | | | | |x| - Datagram with non-unique src address |3.2.2 | | | | |x| Return ICMP error msgs (when not prohibited) |3.3.8 |x| | | | | | | | | | | | Dest Unreachable: | | | | | | | Generate Dest Unreachable (code 2/3) |3.2.2.1 | |x| | | | Pass ICMP Dest Unreachable to higher layer |3.2.2.1 |x| | | | | Higher layer act on Dest Unreach |3.2.2.1 | |x| | | | Interpret Dest Unreach as only hint |3.2.2.1 |x| | | | | Redirect: | | | | | | | Host send Redirect |3.2.2.2 | | | |x| | Update route cache when recv Redirect |3.2.2.2 |x| | | | | Handle both Host and Net Redirects |3.2.2.2 |x| | | | | Discard illegal Redirect |3.2.2.2 | |x| | | | Source Quench: | | | | | | | Send Source Quench if buffering exceeded |3.2.2.3 | | |x| | | Pass Source Quench to higher layer |3.2.2.3 |x| | | | | Higher layer act on Source Quench |3.2.2.3 | |x| | | | Time Exceeded: pass to higher layer |3.2.2.4 |x| | | | | Parameter Problem: | | | | | | | Send Parameter Problem messages |3.2.2.5 | |x| | | | Pass Parameter Problem to higher layer |3.2.2.5 |x| | | | | Report Parameter Problem to user |3.2.2.5 | | |x| | | | | | | | | | ICMP Echo Request or Reply: | | | | | | | Echo server and Echo client |3.2.2.6 |x| | | | |
Echo client |3.2.2.6 | |x| | | |
Discard Echo Request to broadcast address |3.2.2.6 | | |x| | |
Discard Echo Request to multicast address |3.2.2.6 | | |x| | |
Use specific-dest addr as Echo Reply src |3.2.2.6 |x| | | | |
Send same data in Echo Reply |3.2.2.6 |x| | | | |
Pass Echo Reply to higher layer |3.2.2.6 |x| | | | |
Reflect Record Route, Time Stamp options |3.2.2.6 | |x| | | |
Reverse and reflect Source Route option |3.2.2.6 |x| | | | |
| | | | | | |
ICMP Information Request or Reply: |3.2.2.7 | | | |x| |
ICMP Timestamp and Timestamp Reply: |3.2.2.8 | | |x| | |
Minimize delay variability |3.2.2.8 | |x| | | |1
Silently discard b'cast Timestamp |3.2.2.8 | | |x| | |1
Silently discard m'cast Timestamp |3.2.2.8 | | |x| | |1
Use specific-dest addr as TS Reply src |3.2.2.8 |x| | | | |1
Reflect Record Route, Time Stamp options |3.2.2.6 | |x| | | |1
Reverse and reflect Source Route option |3.2.2.8 |x| | | | |1
Pass Timestamp Reply to higher layer |3.2.2.8 |x| | | | |1
Obey rules for "standard value" |3.2.2.8 |x| | | | |1
| | | | | | |
ICMP Address Mask Request and Reply: | | | | | | |
Addr Mask source configurable |3.2.2.9 |x| | | | |
Support static configuration of addr mask |3.2.2.9 |x| | | | |
Get addr mask dynamically during booting |3.2.2.9 | | |x| | |
Get addr via ICMP Addr Mask Request/Reply |3.2.2.9 | | |x| | |
Retransmit Addr Mask Req if no Reply |3.2.2.9 |x| | | | |3
Assume default mask if no Reply |3.2.2.9 | |x| | | |3
Update address mask from first Reply only |3.2.2.9 |x| | | | |3
Reasonableness check on Addr Mask |3.2.2.9 | |x| | | |
Send unauthorized Addr Mask Reply msgs |3.2.2.9 | | | | |x|
Explicitly configured to be agent |3.2.2.9 |x| | | | |
Static config=> Addr-Mask-Authoritative flag |3.2.2.9 | |x| | | |
Broadcast Addr Mask Reply when init. |3.2.2.9 |x| | | | |3
| | | | | | |
ROUTING OUTBOUND DATAGRAMS: | | | | | | |
Use address mask in local/remote decision |3.3.1.1 |x| | | | |
Operate with no gateways on conn network |3.3.1.1 |x| | | | |
Maintain "route cache" of next-hop gateways |3.3.1.2 |x| | | | |
Treat Host and Net Redirect the same |3.3.1.2 | |x| | | |
If no cache entry, use default gateway |3.3.1.2 |x| | | | |
Support multiple default gateways |3.3.1.2 |x| | | | |
Provide table of static routes |3.3.1.2 | | |x| | |
Flag: route overridable by Redirects |3.3.1.2 | | |x| | |
Key route cache on host, not net address |3.3.1.3 | | |x| | |
Include TOS in route cache |3.3.1.3 | |x| | | |
| | | | | | |
Able to detect failure of next-hop gateway |3.3.1.4 |x| | | | |
Assume route is good forever |3.3.1.4 | | | |x| |
Ping gateways continuously |3.3.1.4 | | | | |x| Ping only when traffic being sent |3.3.1.4 |x| | | | | Ping only when no positive indication |3.3.1.4 |x| | | | | Higher and lower layers give advice |3.3.1.4 | |x| | | | Switch from failed default g'way to another |3.3.1.5 |x| | | | | Manual method of entering config info |3.3.1.6 |x| | | | | | | | | | | | REASSEMBLY and FRAGMENTATION: | | | | | | | Able to reassemble incoming datagrams |3.3.2 |x| | | | | At least 576 byte datagrams |3.3.2 |x| | | | | EMTU_R configurable or indefinite |3.3.2 | |x| | | | Transport layer able to learn MMS_R |3.3.2 |x| | | | | Send ICMP Time Exceeded on reassembly timeout |3.3.2 |x| | | | | Fixed reassembly timeout value |3.3.2 | |x| | | | | | | | | | | Pass MMS_S to higher layers |3.3.3 |x| | | | | Local fragmentation of outgoing packets |3.3.3 | | |x| | | Else don't send bigger than MMS_S |3.3.3 |x| | | | | Send max 576 to off-net destination |3.3.3 | |x| | | | All-Subnets-MTU configuration flag |3.3.3 | | |x| | | | | | | | | | MULTIHOMING: | | | | | | | Reply with same addr as spec-dest addr |3.3.4.2 | |x| | | | Allow application to choose local IP addr |3.3.4.2 |x| | | | | Silently discard d'gram in "wrong" interface |3.3.4.2 | | |x| | | Only send d'gram through "right" interface |3.3.4.2 | | |x| | |4 | | | | | | | SOURCE-ROUTE FORWARDING: | | | | | | | Forward datagram with Source Route option |3.3.5 | | |x| | |1 Obey corresponding gateway rules |3.3.5 |x| | | | |1 Update TTL by gateway rules |3.3.5 |x| | | | |1 Able to generate ICMP err code 4, 5 |3.3.5 |x| | | | |1 IP src addr not local host |3.3.5 | | |x| | |1 Update Timestamp, Record Route options |3.3.5 |x| | | | |1 Configurable switch for non-local SRing |3.3.5 |x| | | | |1 Defaults to OFF |3.3.5 |x| | | | |1 Satisfy gwy access rules for non-local SRing |3.3.5 |x| | | | |1 If not forward, send Dest Unreach (cd 5) |3.3.5 | |x| | | |2 | | | | | | | BROADCAST: | | | | | | | Broadcast addr as IP source addr |3.2.1.3 | | | | |x| Receive 0 or -1 broadcast formats OK |3.3.6 | |x| | | | Config'ble option to send 0 or -1 b'cast |3.3.6 | | |x| | | Default to -1 broadcast |3.3.6 | |x| | | | Recognize all broadcast address formats |3.3.6 |x| | | | | Use IP b'cast/m'cast addr in link-layer b'cast |3.3.6 |x| | | | | Silently discard link-layer-only b'cast dg's |3.3.6 | |x| | | | Use Limited Broadcast addr for connected net |3.3.6 | |x| | | |
| | | | | | |
MULTICAST: | | | | | | |
Support local IP multicasting (RFC-1112) |3.3.7 | |x| | | |
Support IGMP (RFC-1112) |3.3.7 | | |x| | |
Join all-hosts group at startup |3.3.7 | |x| | | |
Higher layers learn i'face m'cast capability |3.3.7 | |x| | | |
| | | | | | |
INTERFACE: | | | | | | |
Allow transport layer to use all IP mechanisms |3.4 |x| | | | |
Pass interface ident up to transport layer |3.4 |x| | | | |
Pass all IP options up to transport layer |3.4 |x| | | | |
Transport layer can send certain ICMP messages |3.4 |x| | | | |
Pass spec'd ICMP messages up to transp. layer |3.4 |x| | | | |
Include IP hdr+8 octets or more from orig. |3.4 |x| | | | |
Able to leap tall buildings at a single bound |3.5 | |x| | | |
Footnotes:
(1) Only if feature is implemented.
(2) This requirement is overruled if datagram is an ICMP error message.
(3) Only if feature is implemented and is configured "on".
(4) Unless has embedded gateway functionality or is source routed.
4. TRANSPORT PROTOCOLS 4.1 USER DATAGRAM PROTOCOL -- UDP 4.1.1 INTRODUCTION The User Datagram Protocol UDP [UDP:1] offers only a minimal transport service -- non-guaranteed datagram delivery -- and gives applications direct access to the datagram service of the IP layer. UDP is used by applications that do not require the level of service of TCP or that wish to use communications services (e.g., multicast or broadcast delivery) not available from TCP. UDP is almost a null protocol; the only services it provides over IP are checksumming of data and multiplexing by port number. Therefore, an application program running over UDP must deal directly with end-to-end communication problems that a connection-oriented protocol would have handled -- e.g., retransmission for reliable delivery, packetization and reassembly, flow control, congestion avoidance, etc., when these are required. The fairly complex coupling between IP and TCP will be mirrored in the coupling between UDP and many applications using UDP. 4.1.2 PROTOCOL WALK-THROUGH There are no known errors in the specification of UDP. 4.1.3 SPECIFIC ISSUES 4.1.3.1 Ports UDP well-known ports follow the same rules as TCP well-known ports; see Section 4.2.2.1 below. If a datagram arrives addressed to a UDP port for which there is no pending LISTEN call, UDP SHOULD send an ICMP Port Unreachable message. 4.1.3.2 IP Options UDP MUST pass any IP option that it receives from the IP layer transparently to the application layer. An application MUST be able to specify IP options to be sent in its UDP datagrams, and UDP MUST pass these options to the IP layer.
DISCUSSION:
At present, the only options that need be passed
through UDP are Source Route, Record Route, and Time
Stamp. However, new options may be defined in the
future, and UDP need not and should not make any
assumptions about the format or content of options it
passes to or from the application; an exception to this
might be an IP-layer security option.
An application based on UDP will need to obtain a
source route from a request datagram and supply a
reversed route for sending the corresponding reply.
4.1.3.3 ICMP Messages
UDP MUST pass to the application layer all ICMP error
messages that it receives from the IP layer. Conceptually
at least, this may be accomplished with an upcall to the
ERROR_REPORT routine (see Section 4.2.4.1).
DISCUSSION:
Note that ICMP error messages resulting from sending a
UDP datagram are received asynchronously. A UDP-based
application that wants to receive ICMP error messages
is responsible for maintaining the state necessary to
demultiplex these messages when they arrive; for
example, the application may keep a pending receive
operation for this purpose. The application is also
responsible to avoid confusion from a delayed ICMP
error message resulting from an earlier use of the same
port(s).
4.1.3.4 UDP Checksums
A host MUST implement the facility to generate and validate
UDP checksums. An application MAY optionally be able to
control whether a UDP checksum will be generated, but it
MUST default to checksumming on.
If a UDP datagram is received with a checksum that is non-
zero and invalid, UDP MUST silently discard the datagram.
An application MAY optionally be able to control whether UDP
datagrams without checksums should be discarded or passed to
the application.
DISCUSSION:
Some applications that normally run only across local
area networks have chosen to turn off UDP checksums for
efficiency. As a result, numerous cases of undetected
errors have been reported. The advisability of ever
turning off UDP checksumming is very controversial.
IMPLEMENTATION:
There is a common implementation error in UDP
checksums. Unlike the TCP checksum, the UDP checksum
is optional; the value zero is transmitted in the
checksum field of a UDP header to indicate the absence
of a checksum. If the transmitter really calculates a
UDP checksum of zero, it must transmit the checksum as
all 1's (65535). No special action is required at the
receiver, since zero and 65535 are equivalent in 1's
complement arithmetic.
4.1.3.5 UDP Multihoming
When a UDP datagram is received, its specific-destination
address MUST be passed up to the application layer.
An application program MUST be able to specify the IP source
address to be used for sending a UDP datagram or to leave it
unspecified (in which case the networking software will
choose an appropriate source address). There SHOULD be a
way to communicate the chosen source address up to the
application layer (e.g, so that the application can later
receive a reply datagram only from the corresponding
interface).
DISCUSSION:
A request/response application that uses UDP should use
a source address for the response that is the same as
the specific destination address of the request. See
the "General Issues" section of [INTRO:1].
4.1.3.6 Invalid Addresses
A UDP datagram received with an invalid IP source address
(e.g., a broadcast or multicast address) must be discarded
by UDP or by the IP layer (see Section 3.2.1.3).
When a host sends a UDP datagram, the source address MUST be
(one of) the IP address(es) of the host.
4.1.4 UDP/APPLICATION LAYER INTERFACE
The application interface to UDP MUST provide the full services
of the IP/transport interface described in Section 3.4 of this
document. Thus, an application using UDP needs the functions
of the GET_SRCADDR(), GET_MAXSIZES(), ADVISE_DELIVPROB(), and
RECV_ICMP() calls described in Section 3.4. For example,
GET_MAXSIZES() can be used to learn the effective maximum UDP
maximum datagram size for a particular {interface,remote
host,TOS} triplet.
An application-layer program MUST be able to set the TTL and
TOS values as well as IP options for sending a UDP datagram,
and these values must be passed transparently to the IP layer.
UDP MAY pass the received TOS up to the application layer.
4.1.5 UDP REQUIREMENTS SUMMARY
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-------------------------------------------------|--------|-|-|-|-|-|--
| | | | | | |
UDP | | | | | | |
-------------------------------------------------|--------|-|-|-|-|-|--
| | | | | | |
UDP send Port Unreachable |4.1.3.1 | |x| | | |
| | | | | | |
IP Options in UDP | | | | | | |
- Pass rcv'd IP options to applic layer |4.1.3.2 |x| | | | |
- Applic layer can specify IP options in Send |4.1.3.2 |x| | | | |
- UDP passes IP options down to IP layer |4.1.3.2 |x| | | | |
| | | | | | |
Pass ICMP msgs up to applic layer |4.1.3.3 |x| | | | |
| | | | | | |
UDP checksums: | | | | | | |
- Able to generate/check checksum |4.1.3.4 |x| | | | |
- Silently discard bad checksum |4.1.3.4 |x| | | | |
- Sender Option to not generate checksum |4.1.3.4 | | |x| | |
- Default is to checksum |4.1.3.4 |x| | | | |
- Receiver Option to require checksum |4.1.3.4 | | |x| | |
| | | | | | |
UDP Multihoming | | | | | | |
- Pass spec-dest addr to application |4.1.3.5 |x| | | | |
- Applic layer can specify Local IP addr |4.1.3.5 |x| | | | | - Applic layer specify wild Local IP addr |4.1.3.5 |x| | | | | - Applic layer notified of Local IP addr used |4.1.3.5 | |x| | | | | | | | | | | Bad IP src addr silently discarded by UDP/IP |4.1.3.6 |x| | | | | Only send valid IP source address |4.1.3.6 |x| | | | | UDP Application Interface Services | | | | | | | Full IP interface of 3.4 for application |4.1.4 |x| | | | | - Able to spec TTL, TOS, IP opts when send dg |4.1.4 |x| | | | | - Pass received TOS up to applic layer |4.1.4 | | |x| | |
4.2 TRANSMISSION CONTROL PROTOCOL -- TCP 4.2.1 INTRODUCTION The Transmission Control Protocol TCP [TCP:1] is the primary virtual-circuit transport protocol for the Internet suite. TCP provides reliable, in-sequence delivery of a full-duplex stream of octets (8-bit bytes). TCP is used by those applications needing reliable, connection-oriented transport service, e.g., mail (SMTP), file transfer (FTP), and virtual terminal service (Telnet); requirements for these application-layer protocols are described in [INTRO:1]. 4.2.2 PROTOCOL WALK-THROUGH 4.2.2.1 Well-Known Ports: RFC-793 Section 2.7 DISCUSSION: TCP reserves port numbers in the range 0-255 for "well-known" ports, used to access services that are standardized across the Internet. The remainder of the port space can be freely allocated to application processes. Current well-known port definitions are listed in the RFC entitled "Assigned Numbers" [INTRO:6]. A prerequisite for defining a new well- known port is an RFC documenting the proposed service in enough detail to allow new implementations. Some systems extend this notion by adding a third subdivision of the TCP port space: reserved ports, which are generally used for operating-system-specific services. For example, reserved ports might fall between 256 and some system-dependent upper limit. Some systems further choose to protect well-known and reserved ports by permitting only privileged users to open TCP connections with those port values. This is perfectly reasonable as long as the host does not assume that all hosts protect their low-numbered ports in this manner. 4.2.2.2 Use of Push: RFC-793 Section 2.8 When an application issues a series of SEND calls without setting the PUSH flag, the TCP MAY aggregate the data internally without sending it. Similarly, when a series of segments is received without the PSH bit, a TCP MAY queue the data internally without passing it to the receiving application.
The PSH bit is not a record marker and is independent of
segment boundaries. The transmitter SHOULD collapse
successive PSH bits when it packetizes data, to send the
largest possible segment.
A TCP MAY implement PUSH flags on SEND calls. If PUSH flags
are not implemented, then the sending TCP: (1) must not
buffer data indefinitely, and (2) MUST set the PSH bit in
the last buffered segment (i.e., when there is no more
queued data to be sent).
The discussion in RFC-793 on pages 48, 50, and 74
erroneously implies that a received PSH flag must be passed
to the application layer. Passing a received PSH flag to
the application layer is now OPTIONAL.
An application program is logically required to set the PUSH
flag in a SEND call whenever it needs to force delivery of
the data to avoid a communication deadlock. However, a TCP
SHOULD send a maximum-sized segment whenever possible, to
improve performance (see Section 4.2.3.4).
DISCUSSION:
When the PUSH flag is not implemented on SEND calls,
i.e., when the application/TCP interface uses a pure
streaming model, responsibility for aggregating any
tiny data fragments to form reasonable sized segments
is partially borne by the application layer.
Generally, an interactive application protocol must set
the PUSH flag at least in the last SEND call in each
command or response sequence. A bulk transfer protocol
like FTP should set the PUSH flag on the last segment
of a file or when necessary to prevent buffer deadlock.
At the receiver, the PSH bit forces buffered data to be
delivered to the application (even if less than a full
buffer has been received). Conversely, the lack of a
PSH bit can be used to avoid unnecessary wakeup calls
to the application process; this can be an important
performance optimization for large timesharing hosts.
Passing the PSH bit to the receiving application allows
an analogous optimization within the application.
4.2.2.3 Window Size: RFC-793 Section 3.1
The window size MUST be treated as an unsigned number, or
else large window sizes will appear like negative windows
and TCP will not work. It is RECOMMENDED that
implementations reserve 32-bit fields for the send and
receive window sizes in the connection record and do all
window computations with 32 bits.
DISCUSSION:
It is known that the window field in the TCP header is
too small for high-speed, long-delay paths.
Experimental TCP options have been defined to extend
the window size; see for example [TCP:11]. In
anticipation of the adoption of such an extension, TCP
implementors should treat windows as 32 bits.
4.2.2.4 Urgent Pointer: RFC-793 Section 3.1
The second sentence is in error: the urgent pointer points
to the sequence number of the LAST octet (not LAST+1) in a
sequence of urgent data. The description on page 56 (last
sentence) is correct.
A TCP MUST support a sequence of urgent data of any length.
A TCP MUST inform the application layer asynchronously
whenever it receives an Urgent pointer and there was
previously no pending urgent data, or whenever the Urgent
pointer advances in the data stream. There MUST be a way
for the application to learn how much urgent data remains to
be read from the connection, or at least to determine
whether or not more urgent data remains to be read.
DISCUSSION:
Although the Urgent mechanism may be used for any
application, it is normally used to send "interrupt"-
type commands to a Telnet program (see "Using Telnet
Synch Sequence" section in [INTRO:1]).
The asynchronous or "out-of-band" notification will
allow the application to go into "urgent mode", reading
data from the TCP connection. This allows control
commands to be sent to an application whose normal
input buffers are full of unprocessed data.
IMPLEMENTATION:
The generic ERROR-REPORT() upcall described in Section
4.2.4.1 is a possible mechanism for informing the
application of the arrival of urgent data.
4.2.2.5 TCP Options: RFC-793 Section 3.1 A TCP MUST be able to receive a TCP option in any segment. A TCP MUST ignore without error any TCP option it does not implement, assuming that the option has a length field (all TCP options defined in the future will have length fields). TCP MUST be prepared to handle an illegal option length (e.g., zero) without crashing; a suggested procedure is to reset the connection and log the reason. 4.2.2.6 Maximum Segment Size Option: RFC-793 Section 3.1 TCP MUST implement both sending and receiving the Maximum Segment Size option [TCP:4]. TCP SHOULD send an MSS (Maximum Segment Size) option in every SYN segment when its receive MSS differs from the default 536, and MAY send it always. If an MSS option is not received at connection setup, TCP MUST assume a default send MSS of 536 (576-40) [TCP:4]. The maximum size of a segment that TCP really sends, the "effective send MSS," MUST be the smaller of the send MSS (which reflects the available reassembly buffer size at the remote host) and the largest size permitted by the IP layer: Eff.snd.MSS = min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize where: * SendMSS is the MSS value received from the remote host, or the default 536 if no MSS option is received. * MMS_S is the maximum size for a transport-layer message that TCP may send. * TCPhdrsize is the size of the TCP header; this is normally 20, but may be larger if TCP options are to be sent. * IPoptionsize is the size of any IP options that TCP will pass to the IP layer with the current message. The MSS value to be sent in an MSS option must be less than
or equal to:
MMS_R - 20
where MMS_R is the maximum size for a transport-layer
message that can be received (and reassembled). TCP obtains
MMS_R and MMS_S from the IP layer; see the generic call
GET_MAXSIZES in Section 3.4.
DISCUSSION:
The choice of TCP segment size has a strong effect on
performance. Larger segments increase throughput by
amortizing header size and per-datagram processing
overhead over more data bytes; however, if the packet
is so large that it causes IP fragmentation, efficiency
drops sharply if any fragments are lost [IP:9].
Some TCP implementations send an MSS option only if the
destination host is on a non-connected network.
However, in general the TCP layer may not have the
appropriate information to make this decision, so it is
preferable to leave to the IP layer the task of
determining a suitable MTU for the Internet path. We
therefore recommend that TCP always send the option (if
not 536) and that the IP layer determine MMS_R as
specified in 3.3.3 and 3.4. A proposed IP-layer
mechanism to measure the MTU would then modify the IP
layer without changing TCP.
4.2.2.7 TCP Checksum: RFC-793 Section 3.1
Unlike the UDP checksum (see Section 4.1.3.4), the TCP
checksum is never optional. The sender MUST generate it and
the receiver MUST check it.
4.2.2.8 TCP Connection State Diagram: RFC-793 Section 3.2,
page 23
There are several problems with this diagram:
(a) The arrow from SYN-SENT to SYN-RCVD should be labeled
with "snd SYN,ACK", to agree with the text on page 68
and with Figure 8.
(b) There could be an arrow from SYN-RCVD state to LISTEN
state, conditioned on receiving a RST after a passive
open (see text page 70).
(c) It is possible to go directly from FIN-WAIT-1 to the
TIME-WAIT state (see page 75 of the spec).
4.2.2.9 Initial Sequence Number Selection: RFC-793 Section
3.3, page 27
A TCP MUST use the specified clock-driven selection of
initial sequence numbers.
4.2.2.10 Simultaneous Open Attempts: RFC-793 Section 3.4, page
32
There is an error in Figure 8: the packet on line 7 should
be identical to the packet on line 5.
A TCP MUST support simultaneous open attempts.
DISCUSSION:
It sometimes surprises implementors that if two
applications attempt to simultaneously connect to each
other, only one connection is generated instead of two.
This was an intentional design decision; don't try to
"fix" it.
4.2.2.11 Recovery from Old Duplicate SYN: RFC-793 Section 3.4,
page 33
Note that a TCP implementation MUST keep track of whether a
connection has reached SYN_RCVD state as the result of a
passive OPEN or an active OPEN.
4.2.2.12 RST Segment: RFC-793 Section 3.4
A TCP SHOULD allow a received RST segment to include data.
DISCUSSION
It has been suggested that a RST segment could contain
ASCII text that encoded and explained the cause of the
RST. No standard has yet been established for such
data.
4.2.2.13 Closing a Connection: RFC-793 Section 3.5
A TCP connection may terminate in two ways: (1) the normal
TCP close sequence using a FIN handshake, and (2) an "abort"
in which one or more RST segments are sent and the
connection state is immediately discarded. If a TCP
connection is closed by the remote site, the local
application MUST be informed whether it closed normally or
was aborted.
The normal TCP close sequence delivers buffered data
reliably in both directions. Since the two directions of a
TCP connection are closed independently, it is possible for
a connection to be "half closed," i.e., closed in only one
direction, and a host is permitted to continue sending data
in the open direction on a half-closed connection.
A host MAY implement a "half-duplex" TCP close sequence, so
that an application that has called CLOSE cannot continue to
read data from the connection. If such a host issues a
CLOSE call while received data is still pending in TCP, or
if new data is received after CLOSE is called, its TCP
SHOULD send a RST to show that data was lost.
When a connection is closed actively, it MUST linger in
TIME-WAIT state for a time 2xMSL (Maximum Segment Lifetime).
However, it MAY accept a new SYN from the remote TCP to
reopen the connection directly from TIME-WAIT state, if it:
(1) assigns its initial sequence number for the new
connection to be larger than the largest sequence
number it used on the previous connection incarnation,
and
(2) returns to TIME-WAIT state if the SYN turns out to be
an old duplicate.
DISCUSSION:
TCP's full-duplex data-preserving close is a feature
that is not included in the analogous ISO transport
protocol TP4.
Some systems have not implemented half-closed
connections, presumably because they do not fit into
the I/O model of their particular operating system. On
these systems, once an application has called CLOSE, it
can no longer read input data from the connection; this
is referred to as a "half-duplex" TCP close sequence.
The graceful close algorithm of TCP requires that the
connection state remain defined on (at least) one end
of the connection, for a timeout period of 2xMSL, i.e.,
4 minutes. During this period, the (remote socket,
local socket) pair that defines the connection is busy
and cannot be reused. To shorten the time that a given
port pair is tied up, some TCPs allow a new SYN to be
accepted in TIME-WAIT state.
4.2.2.14 Data Communication: RFC-793 Section 3.7, page 40
Since RFC-793 was written, there has been extensive work on
TCP algorithms to achieve efficient data communication.
Later sections of the present document describe required and
recommended TCP algorithms to determine when to send data
(Section 4.2.3.4), when to send an acknowledgment (Section
4.2.3.2), and when to update the window (Section 4.2.3.3).
DISCUSSION:
One important performance issue is "Silly Window
Syndrome" or "SWS" [TCP:5], a stable pattern of small
incremental window movements resulting in extremely
poor TCP performance. Algorithms to avoid SWS are
described below for both the sending side (Section
4.2.3.4) and the receiving side (Section 4.2.3.3).
In brief, SWS is caused by the receiver advancing the
right window edge whenever it has any new buffer space
available to receive data and by the sender using any
incremental window, no matter how small, to send more
data [TCP:5]. The result can be a stable pattern of
sending tiny data segments, even though both sender and
receiver have a large total buffer space for the
connection. SWS can only occur during the transmission
of a large amount of data; if the connection goes
quiescent, the problem will disappear. It is caused by
typical straightforward implementation of window
management, but the sender and receiver algorithms
given below will avoid it.
Another important TCP performance issue is that some
applications, especially remote login to character-at-
a-time hosts, tend to send streams of one-octet data
segments. To avoid deadlocks, every TCP SEND call from
such applications must be "pushed", either explicitly
by the application or else implicitly by TCP. The
result may be a stream of TCP segments that contain one
data octet each, which makes very inefficient use of
the Internet and contributes to Internet congestion.
The Nagle Algorithm described in Section 4.2.3.4
provides a simple and effective solution to this
problem. It does have the effect of clumping
characters over Telnet connections; this may initially
surprise users accustomed to single-character echo, but
user acceptance has not been a problem.
Note that the Nagle algorithm and the send SWS
avoidance algorithm play complementary roles in
improving performance. The Nagle algorithm discourages
sending tiny segments when the data to be sent
increases in small increments, while the SWS avoidance
algorithm discourages small segments resulting from the
right window edge advancing in small increments.
A careless implementation can send two or more
acknowledgment segments per data segment received. For
example, suppose the receiver acknowledges every data
segment immediately. When the application program
subsequently consumes the data and increases the
available receive buffer space again, the receiver may
send a second acknowledgment segment to update the
window at the sender. The extreme case occurs with
single-character segments on TCP connections using the
Telnet protocol for remote login service. Some
implementations have been observed in which each
incoming 1-character segment generates three return
segments: (1) the acknowledgment, (2) a one byte
increase in the window, and (3) the echoed character,
respectively.
4.2.2.15 Retransmission Timeout: RFC-793 Section 3.7, page 41
The algorithm suggested in RFC-793 for calculating the
retransmission timeout is now known to be inadequate; see
Section 4.2.3.1 below.
Recent work by Jacobson [TCP:7] on Internet congestion and
TCP retransmission stability has produced a transmission
algorithm combining "slow start" with "congestion
avoidance". A TCP MUST implement this algorithm.
If a retransmitted packet is identical to the original
packet (which implies not only that the data boundaries have
not changed, but also that the window and acknowledgment
fields of the header have not changed), then the same IP
Identification field MAY be used (see Section 3.2.1.5).
IMPLEMENTATION:
Some TCP implementors have chosen to "packetize" the
data stream, i.e., to pick segment boundaries when
segments are originally sent and to queue these
segments in a "retransmission queue" until they are
acknowledged. Another design (which may be simpler) is
to defer packetizing until each time data is
transmitted or retransmitted, so there will be no
segment retransmission queue.
In an implementation with a segment retransmission
queue, TCP performance may be enhanced by repacketizing
the segments awaiting acknowledgment when the first
retransmission timeout occurs. That is, the
outstanding segments that fitted would be combined into
one maximum-sized segment, with a new IP Identification
value. The TCP would then retain this combined segment
in the retransmit queue until it was acknowledged.
However, if the first two segments in the
retransmission queue totalled more than one maximum-
sized segment, the TCP would retransmit only the first
segment using the original IP Identification field.
4.2.2.16 Managing the Window: RFC-793 Section 3.7, page 41
A TCP receiver SHOULD NOT shrink the window, i.e., move the
right window edge to the left. However, a sending TCP MUST
be robust against window shrinking, which may cause the
"useable window" (see Section 4.2.3.4) to become negative.
If this happens, the sender SHOULD NOT send new data, but
SHOULD retransmit normally the old unacknowledged data
between SND.UNA and SND.UNA+SND.WND. The sender MAY also
retransmit old data beyond SND.UNA+SND.WND, but SHOULD NOT
time out the connection if data beyond the right window edge
is not acknowledged. If the window shrinks to zero, the TCP
MUST probe it in the standard way (see next Section).
DISCUSSION:
Many TCP implementations become confused if the window
shrinks from the right after data has been sent into a
larger window. Note that TCP has a heuristic to select
the latest window update despite possible datagram
reordering; as a result, it may ignore a window update
with a smaller window than previously offered if
neither the sequence number nor the acknowledgment
number is increased.
4.2.2.17 Probing Zero Windows: RFC-793 Section 3.7, page 42 Probing of zero (offered) windows MUST be supported. A TCP MAY keep its offered receive window closed indefinitely. As long as the receiving TCP continues to send acknowledgments in response to the probe segments, the sending TCP MUST allow the connection to stay open. DISCUSSION: It is extremely important to remember that ACK (acknowledgment) segments that contain no data are not reliably transmitted by TCP. If zero window probing is not supported, a connection may hang forever when an ACK segment that re-opens the window is lost. The delay in opening a zero window generally occurs when the receiving application stops taking data from its TCP. For example, consider a printer daemon application, stopped because the printer ran out of paper. The transmitting host SHOULD send the first zero-window probe when a zero window has existed for the retransmission timeout period (see Section 4.2.2.15), and SHOULD increase exponentially the interval between successive probes. DISCUSSION: This procedure minimizes delay if the zero-window condition is due to a lost ACK segment containing a window-opening update. Exponential backoff is recommended, possibly with some maximum interval not specified here. This procedure is similar to that of the retransmission algorithm, and it may be possible to combine the two procedures in the implementation. 4.2.2.18 Passive OPEN Calls: RFC-793 Section 3.8 Every passive OPEN call either creates a new connection record in LISTEN state, or it returns an error; it MUST NOT affect any previously created connection record. A TCP that supports multiple concurrent users MUST provide an OPEN call that will functionally allow an application to LISTEN on a port while a connection block with the same local port is in SYN-SENT or SYN-RECEIVED state. DISCUSSION:
Some applications (e.g., SMTP servers) may need to
handle multiple connection attempts at about the same
time. The probability of a connection attempt failing
is reduced by giving the application some means of
listening for a new connection at the same time that an
earlier connection attempt is going through the three-
way handshake.
IMPLEMENTATION:
Acceptable implementations of concurrent opens may
permit multiple passive OPEN calls, or they may allow
"cloning" of LISTEN-state connections from a single
passive OPEN call.
4.2.2.19 Time to Live: RFC-793 Section 3.9, page 52
RFC-793 specified that TCP was to request the IP layer to
send TCP segments with TTL = 60. This is obsolete; the TTL
value used to send TCP segments MUST be configurable. See
Section 3.2.1.7 for discussion.
4.2.2.20 Event Processing: RFC-793 Section 3.9
While it is not strictly required, a TCP SHOULD be capable
of queueing out-of-order TCP segments. Change the "may" in
the last sentence of the first paragraph on page 70 to
"should".
DISCUSSION:
Some small-host implementations have omitted segment
queueing because of limited buffer space. This
omission may be expected to adversely affect TCP
throughput, since loss of a single segment causes all
later segments to appear to be "out of sequence".
In general, the processing of received segments MUST be
implemented to aggregate ACK segments whenever possible.
For example, if the TCP is processing a series of queued
segments, it MUST process them all before sending any ACK
segments.
Here are some detailed error corrections and notes on the
Event Processing section of RFC-793.
(a) CLOSE Call, CLOSE-WAIT state, p. 61: enter LAST-ACK
state, not CLOSING.
(b) LISTEN state, check for SYN (pp. 65, 66): With a SYN
bit, if the security/compartment or the precedence is
wrong for the segment, a reset is sent. The wrong form
of reset is shown in the text; it should be:
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
(c) SYN-SENT state, Check for SYN, p. 68: When the
connection enters ESTABLISHED state, the following
variables must be set:
SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK
(d) Check security and precedence, p. 71: The first heading
"ESTABLISHED STATE" should really be a list of all
states other than SYN-RECEIVED: ESTABLISHED, FIN-WAIT-
1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, and
TIME-WAIT.
(e) Check SYN bit, p. 71: "In SYN-RECEIVED state and if
the connection was initiated with a passive OPEN, then
return this connection to the LISTEN state and return.
Otherwise...".
(f) Check ACK field, SYN-RECEIVED state, p. 72: When the
connection enters ESTABLISHED state, the variables
listed in (c) must be set.
(g) Check ACK field, ESTABLISHED state, p. 72: The ACK is a
duplicate if SEG.ACK =< SND.UNA (the = was omitted).
Similarly, the window should be updated if: SND.UNA =<
SEG.ACK =< SND.NXT.
(h) USER TIMEOUT, p. 77:
It would be better to notify the application of the
timeout rather than letting TCP force the connection
closed. However, see also Section 4.2.3.5.
4.2.2.21 Acknowledging Queued Segments: RFC-793 Section 3.9
A TCP MAY send an ACK segment acknowledging RCV.NXT when a
valid segment arrives that is in the window but not at the
left window edge.
DISCUSSION:
RFC-793 (see page 74) was ambiguous about whether or
not an ACK segment should be sent when an out-of-order
segment was received, i.e., when SEG.SEQ was unequal to
RCV.NXT.
One reason for ACKing out-of-order segments might be to
support an experimental algorithm known as "fast
retransmit". With this algorithm, the sender uses the
"redundant" ACK's to deduce that a segment has been
lost before the retransmission timer has expired. It
counts the number of times an ACK has been received
with the same value of SEG.ACK and with the same right
window edge. If more than a threshold number of such
ACK's is received, then the segment containing the
octets starting at SEG.ACK is assumed to have been lost
and is retransmitted, without awaiting a timeout. The
threshold is chosen to compensate for the maximum
likely segment reordering in the Internet. There is
not yet enough experience with the fast retransmit
algorithm to determine how useful it is.
(page 95 continued on part 5)