Internet DRAFT - draft-davie-stt
draft-davie-stt
Network Working Group B. Davie, Ed.
Internet-Draft J. Gross
Intended status: Informational VMware, Inc.
Expires: October 30, 2016 April 28, 2016
A Stateless Transport Tunneling Protocol for Network Virtualization
(STT)
draft-davie-stt-08
Abstract
Network Virtualization places unique requirements on tunneling
protocols. This draft describes STT (Stateless Transport Tunneling),
a tunnel encapsulation that enables overlay networks to be built in
virtualized networks. STT is particularly useful when some tunnel
endpoints are in end-systems, as it utilizes the capabilities of the
network interface card to improve performance. This draft documents
the protocol and the rationale for its design, and highlights issues
that may arise in deployments.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 30, 2016.
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Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Reference Model . . . . . . . . . . . . . . . . . . . . . 5
2. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Segmentation Offload . . . . . . . . . . . . . . . . . . 6
2.2. Metadata . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Context Information . . . . . . . . . . . . . . . . . . . 7
2.4. Alignment . . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Equal Cost Multipath . . . . . . . . . . . . . . . . . . 8
2.6. Efficient Software Processing . . . . . . . . . . . . . . 8
3. Frame Formats . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. STT Frame Format . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Handling non-TCP/IP and non-UDP/IP payloads . . . . . 12
3.2. Usage of TCP Header by STT . . . . . . . . . . . . . . . 13
3.3. Encapsulation of STT Segments in IP . . . . . . . . . . . 15
3.3.1. Diffserv and ECN-Marking . . . . . . . . . . . . . . 15
3.3.2. Packet Loss . . . . . . . . . . . . . . . . . . . . . 15
3.4. Broadcast and Multicast . . . . . . . . . . . . . . . . . 16
4. Interoperability Issues . . . . . . . . . . . . . . . . . . . 16
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Network Virtualization places unique requirements on tunneling
protocols. The utility of tunneling in virtualized data centers has
been described elsewhere; see, for example [RFC7364], [VL2],
[RFC7348], [RFC7637], [I-D.ietf-nvo3-geneve]. Tunneling allows a
virtual overlay topology to be constructed on top of the physical
data center network, and provides benefits such as:
o Ability to manage overlapping addresses between multiple tenants
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o Decoupling of the virtual topology provided by the tunnels from
the physical topology of the network
o Support for virtual machine mobility independent of the physical
network
o Support for essentially unlimited numbers of virtual networks (in
contrast to VLANs, for example)
o Decoupling of the network service provided to servers from the
technology used in the physical network (e.g. providing an L2
service over an L3 fabric)
o Isolating the physical network from the addressing of the virtual
networks, thus avoiding issues such as MAC table size in physical
switches.
This draft describes STT (Stateless Transport Tunneling), a tunnel
encapsulation that enables overlay networks to be built in
virtualized data center networks, providing the benefits outlined
above. STT is particularly useful when some tunnel endpoints are in
end-systems, as it utilizes the capabilities of standard network
interface cards to improve performance. Multiple independent
implementations of STT exist and are in production use.
STT is an IP-based encapsulation and utilizes a TCP-like header
inside the IP header. It is, however, stateless, i.e., there is no
TCP connection state of any kind associated with the tunnel. The
TCP-like header is used for pragmatic reasons, to leverage the
capabilities of existing network interface cards, but should not be
interpreted as implying any sort of connection state between
endpoints.
STT is typically used to carry Ethernet frames between tunnel
endpoints. These frames may be considerably larger than the MTU of
the physical network - up to 64KB. Fields in the tunnel header are
used to allow these large frames to be segmented at the entrance to
the tunnel according to the MTU of the physical network and
subsequently reassembled at the far end of the tunnel.
Because STT uses TCP's header format and protocol number (6), some
care needs to be taken in the deployment of STT. Section 4 describes
these deployment considerations.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.2. Terminology
The following terms are used in this document:
Stateless Transport Tunneling (STT). The tunneling mechanism defined
in this document. The name derives from the fact that the tunnel
header resembles the TCP/IP headers (hence "transport" tunneling)
while "stateless" refers to the fact that none of the normal TCP
state (connection state, send and receive windows, congestion state
etc.) is associated with the tunnel (as would be required if an
actual TCP connection were used for tunneling).
STT Frame. The unit of data that is passed into the tunnel prior to
segmentation and encapsulation. This frame typically consists of an
Ethernet frame and an STT Frame header. These frames may be up to
64KB in size.
STT Segment. The unit of data that is transmitted on the underlay
network over which the tunnel operates. An STT segment has headers
that are syntactically the same as the TCP/IP headers, and typically
contains part of an STT frame as the payload. These segments must
fit within the MTU of the physical network.
Context ID. A 64-bit field in the STT frame header that conveys
information about the disposition of the STT frame between the tunnel
endpoints. One example use of the Context ID is to direct delivery
of the STT frame payload to the appropriate virtual network or
virtual machine.
MSS. Maximum Segment Size. The maximum number of bytes that can be
sent in one TCP segment [RFC0793].
NIC. Network Interface Card.
TSO. TCP Segmentation Offload. A function provided by many
commercial NICs that allows large data units to be passed to the NIC,
the NIC being responsible for creating MSS-sized segments with
correct TCP/IP headers.
LRO. Large Receive Offload. The receive-side equivalent function of
TSO, in which multiple TCP segments are coalesced into larger data
units.
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VM. Virtual Machine.
1.3. Reference Model
Our conceptual model for a virtualized network is shown in Figure 1.
STT tunnels extend in this figure from one virtual switch to another,
providing a virtual link between the switches over some arbitrary
underlay. More generally, STT tunnels operate between a pair of
tunnel endpoints; these endpoints may be virtual switches, physical
switches, or some other device (e.g. an appliance). The STT tunnel
provides a virtual point-to-point Ethernet link between the
endpoints. Frames are handed to the tunnel by some entity (e.g. a VM
that is connected to a virtual switch in this picture) and first
encapsulated with an STT Frame header. STT Frames may then be
fragmented in the NIC, and are encapsulated with a tunnel header (the
STT segment header) for transmission over the underlay. Note that
other models are possible, e.g., where one or both tunnel endpoints
are implemented in a physical switch. In such cases the tunnel
endpoint may forward packets to and from another link (physical or
virtual) rather than to a VM.
+----------------------+ +----------------------+
| +--+ +-------+---+ | | +---+-------+ +--+ |
| |VM|---| | | | | | | |---|VM| |
| +--+ |Virtual|NIC|--- Underlay --- |NIC|Virtual| +--+ |
| +--+ |Switch | | | Network | | |Switch | +--+ |
| |VM|---| | | | | | | |---|VM| |
| +--+ +-------+---+ | | +---+-------+ +--+ |
+----------------------+ +----------------------+
()===============================()
Switch-Switch tunnel
Figure 1: STT Reference Model
2. Design Rationale
We take as given the need for some form of tunneling to support the
virtualization of the network as described in Section 1. One might
reasonably ask whether some existing tunneling protocol such as
GRE[RFC2784] or L2TPv3[RFC3931] might suffice. In fact, [RFC7637]
does just that, using GRE. The primary motivation for STT as opposed
to one of the existing tunneling methods is to improve the
performance of data transfers from hosts that implement tunnel
endpoints. We expand on this rationale below.
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2.1. Segmentation Offload
A large percentage of network interface cards (NICs) in use today are
able to perform TCP segmentation offload (TSO). When a NIC supports
TSO, the host hands a large (greater than 1 TCP MSS) frame of data to
the NIC along with a set of metadata which includes, among other
things, the desired MSS, and various fields needed to complete the
TCP header. The NIC fragments the frame into MSS-sized segments,
performs the TCP Checksum operation, and applies the appropriate
headers (TCP, IP and MAC) to each segment.
On the receive side, some NICs support the reassembly of TCP
segments, a function referred to as large receive offload (LRO). In
this case, NICs attempt to reassemble TCP segments and pass larger
aggregates of data to the host. (Since TCP's service model is a byte
stream, there is no higher level frame for the NIC to reassemble, but
it can pass chunks of the stream larger than one MSS to the host.)
The benefits to the host include fewer per-packet operations and
larger data transfers between host and NIC, which amortizes the per-
transfer cost (such as interrupt processing) more efficiently. These
gains can translate into significant performance gains for data
transfer from the host to the network.
STT is explicitly designed to leverage the TSO capabilities of
currently available NICs. While one might think of segmentation as a
generic function, the majority of NICs are designed specifically to
support TCP segmentation offload, as the details of the segmentation
function are highly dependent on the specifics of TCP. In order to
leverage such capability, therefore, the STT segment header is
syntactically identical to a valid TCP header. However, we use some
of the fields in the TCP header (specifically, sequence number and
ACK number) to support the objectives of STT. The details are
described in Section 3.2. In essence, we need the same set of
information that IP datagrams carry when IP fragmentation takes
place: a unique identifier for the frame that has been fragmented, an
offset into that frame for the current fragment, and the length of
the frame to be reassembled. We fit these fields into the TCP header
fields traditionally used for the SEQ and ACK numbers. STT segments
are transmitted as IP datagrams using the TCP protocol number (6).
The primary means to recognize STT segments is the destination port
number. We discuss the interoperability impact of these design
choices in Section 4.
The net effect of using TSO is that the frame size that is sent by
endpoints in the virtualized network can be much larger than the MTU
of the underlying physical network. The primary benefit of this is a
significant performance gain when large amounts of data are being
transferred between nodes in the virtual network. A secondary effect
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is that the header of the STT frame is amortized across a larger
amount of data, reducing the need to shrink the STT frame header to
minimum size.
Note that, while segmentation offload is the primary NIC function
that STT takes advantage of, other NIC offload functions such as
checksum calculation can also be leveraged.
2.2. Metadata
When a frame is delivered to the NIC that supports TSO for
segmentation and transmission, a certain amount of metadata is
typically passed along with it. This includes the MSS and
potentially a VLAN tag to be applied to the transmitted packets.
In some virtualized network deployments, an STT frame may traverse a
tunnel, be received and reassembled at an STT endpoint, and then be
sent on another physical interface. In such cases, the tunnel
terminating endpoint may need to pass metadata to a NIC to enable
transmission of frames on the physical link. For this reason,
appropriate metadata is carried in the STT frame header.
2.3. Context Information
When an STT Frame is received by a tunnel endpoint, it needs to be
directed to the appropriate entity in the virtualized network to
which it belongs. For this reason, a Context ID is required in the
STT frame header. Some other encapsulations (e.g. [RFC7348],
[RFC7637]) use an explicit tenant network identifier or virtual
network identifier. The Context Identifier can be thought of as a
generalized form of virtual network identifier. Using a larger and
more general identifier allows for a broader range of service models
and allows ample room for future expansion. There is little downside
to using a larger field here because it is amortized across the
entire STT Frame rather than being present in each packet.
2.4. Alignment
Software implementations of tunnel endpoints benefit from 32-bit
alignment of the data to be manipulated. Because the Ethernet header
is not a multiple of 32-bits (it is 14 bytes), 2 bytes of padding are
added to the STT header, causing the payload beyond the encapsulated
Ethernet header, which typically includes the IP header of the
encapsulated frame, to be 32-bit aligned.
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2.5. Equal Cost Multipath
It is essential that traffic passing through the physical network can
be efficiently distributed across multiple paths. Standard equal
cost multipath (ECMP) techniques involve hashing on address and port
numbers in the outer protocol headers. There are two main issues to
address with ECMP. First, it is important that, when a set of
packets belong to a single flow (e.g. a TCP connection in the virtual
network), all those packets should follow the same path. Second, all
paths should be used efficiently, i.e. there needs to be sufficient
entropy among the different flows to ensure they get distributed
evenly across multiple paths.
STT achieves the first goal by ensuring that the source and
destination ports and addresses in the outer header are all the same
for a single flow. The second goal is achieved by generating the
source port using a random hash of fields in the headers of the inner
packets, e.g. the ports and addresses of the virtual flow's packets.
We provide more details on the usage of port numbers in Section 3.2.
2.6. Efficient Software Processing
The design of STT is largely motivated by the desire to tunnel
packets efficiently between virtual switches running in software. In
addition to the points noted above, this leads to some design
optimizations to simplify processing of packets, such as the use of
an "L4 offset" field in the STT header to enable the payload to be
located quickly without extensive header parsing.
3. Frame Formats
STT encapsulates data payloads of up to 64KB (limited by the length
field in the STT segment header, described in Section 3.2). Those
frames are then handed to the NIC, which segments them to an
appropriate size given the MTU of the underlying physical network,
and encapsulates the resulting segments in a TCP-like header, which
in turn is encapsulated by an IP header and finally a MAC header.
(The header is "TCP-like" in the sense that it has all the same
fields as a standard TCP header, but some are interpreted differently
as described in Section 3.2.) The encapsulation process is
illustrated in Figure 2.
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+-----------+ +----------+ +----------+
| IP Header | |IP Header | |IP header |
+-----------+ +-----------+ +----------+ +----------+
|STT Frame | |TCP-like | |TCP-like | |TCP-like |
| Header | | header | | header | | header |
+-----------+ +-----------+ +----------+ +----------+
| | ---> | STT Frame | |Next part | ... |Last part |
|Payload | | Header | |of Payload| |of Payload|
. . +-----------+ | | | |
. . | | | | | |
. . | Start of | | | | |
+-----------+ | Payload | | | +----------+
+-----------+ +----------+
Original data STT Frame is segmented by the NIC and
frame is encapped transmitted as a set of TCP segments (MAC
with STT Header headers not shown)
Figure 2: STT Frame Fragments and Encapsulation
The details of the STT Frame header and the usage of the TCP-like
header are described in detail below. The TCP segments shown in
Figure 2 are of course further encapsulated as IP datagrams, and may
be sent as either IPv4 or IPv6. The resulting IP datagrams are then
transmitted in the appropriate MAC level frame (e.g. Ethernet, not
shown in the figure) for the underlying physical network over which
the tunnels are established.
3.1. STT Frame Format
Figure 3 illustrates the header of an STT frame before it is
segmented.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Flags | L4 Offset | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max. Segment Size | PCP |V| VLAN ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Context ID (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding | data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
Figure 3: STT Frame Format
The STT frame header contains the following fields:
o Version - currently 0. If a non-zero version field is received by
an implementation that supports only version zero, the frame MUST
be discarded.
o Flags - describes encapsulated packet, see below.
o L4 offset - offset in bytes from the end of the STT Frame header
to the start of the encapsulated layer 4 (TCP/UDP) header. If the
encapsulated packet is not IPv4 or IPv6, this field SHOULD be set
to zero.
o Reserved field - MUST be zero on transmission and ignored on
receipt.
o Max Segment Size - the segment size (the negotiated MSS in the
case of TCP) that should be used by a tunnel endpoint that is
transmitting this frame onto another network. MUST be zero if
segmentation offload is not in use.
o PCP - the 3-bit Priority Code Point field that should be applied
to this packet by an STT tunnel endpoint on transmission to
another network (see Section 2.2). Meaningful only if the V bit
is set.
o V - a one bit flag that, if set, indicates the presence of a valid
VLAN ID in the following field and valid PCP in the preceding
field. When this flag is set, an 802.1Q header will be applied to
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the packet by the STT tunnel endpoint on transmission. The TPID
will be 0x8100.
o VLAN ID - 12-bit VLAN tag that should be applied to this packet by
an STT tunnel endpoint on transmission to another network (see
Section 2.2). Any valid VLAN ID (including zero) may be used.
Meaningful only if the V bit is set.
o Context ID - 64 bits of context information, described in detail
in Section 2.3.
o Padding - 16 bits as described in Section 2.4. MUST be set to
zero on transmission and ignored on receipt.
The flags field is an 8-bit field organized as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|C|P|V|T| Res. |
+-+-+-+-+-+-+-+-+
Figure 4: STT Flags
The meanings of the flags is as follows:
o C: Checksum verified. Set if the checksum of the encapsulated
packet has been verified by the sender.
o P: Checksum partial. Set if the checksum in the encapsulated
packet has been computed only over the TCP/IP pseudoheader (or
UDP/IP pseudoheader, if the encapsulated packet is UDP). This bit
MUST be set if segmentation offload is used by the sender. Note
that bit 0 and bit 1 cannot both be set in the same header.
o V: IP version. Set if the encapsulated packet is IPv4, not set if
the packet is IPv6. See below for discussion of non-IP payloads.
o T: TCP payload. Set if the encapsulated packet is TCP.
o Bits 4 through 7 are reserved and MUST be zero on transmission and
ignored on receipt.
As noted above, several of these fields are present primarily to
enable efficient processing of the packet when it is received at a
tunnel endpoint. (For example, it's entirely possible to determine
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if the packet is IPv4 or IPv6 by looking at the Ethernet header -
it's just more efficient not to have to do so.)
The payload of the STT frame is an untagged Ethernet frame.
In the case where the Ethernet frame contains TCP/IP or UDP/IP as its
payload, this encapsulated packet should be correctly formatted as if
it were about to undergo unfragmented transmission (even though it
will ultimately be segmented as part of the transmission process).
This means it should have a correct TCP or UDP checksum (possibly
"partial", as noted above), correct length fields for its
unfragmented state, and correct IP header checksum (if IPv4).
If the length of the payload to be encapsulated exceeds 64KB, or if
the offset to the L4 header exceeds 255 bytes, then it will not be
possible to offload the packet to the NIC for segmentation. In this
case, the payload needs to be segmented and checksummed before being
encapsulated in STT frames.
Because there is no negotiation between end-points of an STT tunnel,
only basic TSO capabilities should be assumed. For example, ECN
(explicit congestion notification) support should not be assumed, so
TSO should not be requested for packets requiring such support.
Instead, such payloads should be segmented before being encapsulated
in STT frames.
3.1.1. Handling non-TCP/IP and non-UDP/IP payloads
Note that the STT header does not have a general "protocol" field to
allow the efficient processing of arbitrary payloads. The current
version is designed to provide a virtual Ethernet link, and hence
efficiently supports only Ethernet frames as the payload. The
Ethernet header itself contains a protocol field, which then
identifies the higher layer protocol, so it is straightforward to
accommodate non-IP traffic. Note however that offloading support
will not typically be available for traffic other than the following:
TCP and UDP over IPv4 or IPv6, with a maximum of a single VLAN tag
stored in the STT header. Other protocols will need to be
appropriately formatted for direct transmission prior to
encapsulation.
It will be noted that the STT Frame header does contain fields that
are intended to assist in efficient processing of IPv4 and IPv6
packets. These fields MUST be set to zero and ignored on receipt for
packets not being offloaded.
The use of STT to carry payloads other than Ethernet is theoretically
possible but is beyond the scope of this document.
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3.2. Usage of TCP Header by STT
Figure 5 illustrates the usage of the TCP header by STT. This figure
is essentially identical to that in [RFC0793] with the exception that
we denote with an asterisk (*) two fields that are used by STT to
convey something other than the information that is conveyed by TCP.
Syntactically, STT segments look identical to TCP segments. However,
STT tunnel endpoints treat the Sequence number and Acknowledgment
number differently than TCP endpoints treat those fields.
Furthermore, as noted above, there is no TCP state machine associated
with an STT tunnel.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number(*) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number(*) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |U|A|P|R|S|F| |
| Offset| Reserved |R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: STT Segment Format
The Destination port, assigned by IANA, is 7471.
In order to allow correct reassembly of the STT frame, the source
port MUST be constant for all segments of a single STT frame.
As noted above (Section 2.5) the source port SHOULD be the same for
all frames that belong to a single flow in the virtual network, e.g.
a single TCP connection.
Also, to encourage efficient distribution of traffic among multiple
paths when ECMP is used, the method to calculate the source port
should provide a random distribution of source port numbers. An
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example mechanism would be a random hash on ports and addresses of
the TCP headers of the flow in the virtual network.
The Sequence number and Acknowledgment number fields are re-purposed
in a way that does not confuse NICs that expect them to be used in
the conventional manner. The ACK field is used as a packet
identifier for the purposes of fragmentation, equivalent in function
to the Identification field of IPv4 or the IPv6 Fragment header: it
MUST be constant for all STT segments of a given frame, and different
from any value used recently for other STT frames sent over this
tunnel. ("Recent" in this context means a long enough interval that
packets from the frame that last used this value of the ACK field
should have all been delivered. Similar considerations apply to the
reuse of the IP Fragment Identifier, discussed in [RFC6864], but note
that packet lifetimes in a data center are likely to be relatively
short.)
The upper 16 bits of the the SEQ field are used to convey the length
of the STT frame in bytes. The lower 16 bits of the SEQ field are
used to convey the offset (in bytes) of the current fragment within
the larger STT frame. The task of updating the SEQ field on each
transmitted segment is the responsibility of the NIC.
Reassembly of the fragments may be done partially by NICs that
perform LRO, since the sequence numbers of frames will increment
appropriately. That is, the upper 16 bits don't change, and the
lower 16 bits increment by N for every N byte segment that is
transmitted, just as would be the case if an actual sequence number
were being sent. Note that the size limit of an STT frame ensures
that sequence numbers cannot wrap while sending the segments of a
single STT frame.
Many NICs, when performing LRO, will only merge packets with the same
ACK value. In the event that a NIC does not require the ACK field to
be constant when merging received packets, LRO MUST be disabled for
this NIC when using STT. In this case, STT frame reassembly will be
the responsibility of the software on the receiving host.
All the fields after ACK have their conventional meaning, although
nothing will be done with the Window or Urgent pointer values. Those
fields SHOULD be zero on transmit and ignored on receipt. It is
RECOMMENDED that the PSH (Push) flag be set when transmitting the
last segment of a frame in order to cause data to be delivered by the
NIC without waiting for other fragments. The ACK flag SHOULD be set
to ensure that a receiving NIC examines the ACK field. All other
flags SHOULD be zero on transmit and ignored on receipt.
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3.3. Encapsulation of STT Segments in IP
From the perspective of IP, an STT segment is just like any other TCP
segment. The protocol number (IPv4) or Next Header (IPv6) has the
value 6, as for regular TCP. The resulting IP datagram is then
encapsulated in the appropriate L2 header (e.g. Ethernet) for
transmission on the physical medium.
3.3.1. Diffserv and ECN-Marking
When traffic is encapsulated in a tunnel header, there are numerous
options as to how the Diffserv Code-Point (DSCP) and ECN markings are
set in the outer header and propagated to the inner header on
decapsulation.
[RFC2983] defines two modes for mapping the DSCP markings from inner
to outer headers and vice versa. The Uniform model copies the inner
DSCP marking to the outer header on tunnel ingress, and copies that
outer header value back to the inner header at tunnel egress. The
Pipe model sets the DSCP value to some value based on local policy at
ingress and does not modify the inner header on egress. Both models
SHOULD be supported by STT endpoints. However, there is an
additional complexity with the uniform model for STT, because a
single IP datagram that is transmitted over the tunnel appears as
multiple IP datagrams on the wire. Thus it is not guaranteed that
all segments of the STT frame will have the same DSCP at egress. If
uniform model behavior is configured, it is RECOMMENDED that the DSCP
of the first segment of the STT frame be used to set the DSCP value
of the IP header in the decapsulated STT frame.
[RFC6040] describes the correct ECN behavior for any type of IP in IP
tunnel, and this behavior SHOULD be followed for STT tunnels. As
with the Uniform Diffserv tunnel model, the fact that one inner IP
datagram is segmented into multiple outer datagrams makes the
situation slightly more complex. It is RECOMMENDED that if any
segment of the received STT frame has the CE (congestion experienced)
bit set in its IP header, then the CE bit SHOULD be set in the IP
header of the decapsulated STT frame.
3.3.2. Packet Loss
Individual IP datagrams may be dropped (most often due to congestion)
and, since there is no acknowledgment or reliable delivery of these
datagrams, there is the potential to corrupt an entire STT Frame due
to the loss of a single IP datagram. The negative consequences of
such partial losses have been known for many years (see, for example,
[KM87]). Fortunately, there are solutions to this problem in the
case where the higher layer protocol running over STT is TCP. An STT
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receiving endpoint running in an end-system, as shown in Figure 1 for
example, is not required to deliver complete STT frames to the TCP
stack in the receiving VM. A partial frame payload can be delivered
and the receiving TCP stack can deal with the missing bytes just as
it would if running directly over a physical network. That is, TCP
in the VM can send ACKs for the contiguous bytes received to trigger
retransmission of the missing bytes by the sender. This is similar
to the operation of LRO in current NICs. There are some subtleties
to making this work correctly in the STT context, and it does depend
on the STT endpoint being aware of the higher layer protocols
consuming data in the VM to which it is connected. The main point of
this discussion is that, in the common deployments of STT running in
a virtual switch, the potential harm of losing individual packets is
not as serious as it might first appear.
3.4. Broadcast and Multicast
It is possible to establish point-to-multipoint STT tunnels by using
an IP multicast address as the destination address of the tunnel.
These may be used for broadcast or multicast traffic if the
underlying physical network supports IP multicast. Control
mechanisms for setting up such multicast groups are beyond the scope
of this document. It is worth repeating that, despite the syntactic
resemblance between the STT segment header and the TCP header, there
is no TCP state machine associated with an STT tunnel, so the
traditional issues of combining multicast with TCP (or reliable
transports more generally) do not arise.
4. Interoperability Issues
It will be noted that an STT packet on the wire appears exactly the
same as a TCP packet, but that processing of an STT packet on
reception is entirely different from TCP - no three-way handshake to
establish a connection, no ACKs, retransmission, etc. Hence, an STT
tunnel endpoint clearly needs to be configured to behave in the
correct manner rather than to perform standard TCP processing on the
packet. The primary way to recognize an STT segment is the
destination port number in the TCP header. In the event that an STT
packet is inadvertently delivered to a device that is not configured
to behave as an STT tunnel endpoint, no TCP connection will be
established and STT packets will be dropped.
Being stateless, STT does not provide any sort of congestion control.
In this sense it is equivalent to other tunneling protocols such as
GRE. The assumption is that congestion control, if required, is
provided by higher layers (e.g. a real TCP connection generating the
payloads of STT frames), just as in any other tunneling protocol.
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STT deployments are almost entirely limited at present to intra-data
center environments. In these environments, STT tunnels between
pairs of endpoints are typically created by some sort of network
virtualization controller. STT packets should therefore remain
within the perimeter of the overlay that is managed by that
controller. In the event of some misconfiguration or erroneous
controller behavior, STT packets could be sent outside of this
controlled domain into the broader Internet. As noted above, any
endpoint that is not expecting STT packets will drop them, as they
will appear to belong to an unestablished TCP session. Many
firewalls are also likely to drop erroneously sent STT packets for
the same reason.
Within a network virtualization overlay, there may be middle boxes
(e.g. firewalls) that process TCP. It is likely that, in the near
term at least, such devices will drop STT packets, as there will be
no TCP connection state established. This could prevent the correct
operation of the overlay. This is clearly undesirable, but it is a
general issue with any form of tunneling - the nature of many middle
boxes is that they will not permit tunnels to pass through them.
Hence the best solution is simply to avoid deploying middle boxes at
locations where STT tunnels (or other forms of tunnels for network
virtualization) will need to pass through them. This will not,
however, always be feasible, especially when virtualized networks
extend among multiple data centers. Other solutions include
configuring the middle boxes to permit TCP packets to pass through
when the port number matches the port assigned for STT. In this case
the middle boxes would have to permit the packets to pass in spite of
the lack of an established TCP connection and the repurposing of the
SEQ and ACK fields.
In the longer term, we might reasonably expect that middle boxes
would be able to recognize STT traffic, and to terminate and
originate STT tunnels if necessary (e.g. to perform functions that
require the STT payload to be inspected such as stateful
firewalling).
It is also possible to provide all the functionality of STT using a
different IP protocol number (or next header value in IPv6). This
approach could make sense in the long run but will typically not
enable current NIC hardware to be leveraged for TSO and LRO
functions. An alternative approach is to move to a UDP-based
encapsulation such as Geneve [I-D.ietf-nvo3-geneve]. This, too,
requires NICs to evolve to support TSO and LRO on tunneled traffic.
It is also possible to run STT traffic over other forms of tunnel
(GRE, IPSEC, etc.) in which case the STT traffic can pass through
appropriately configured middle boxes.
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5. IANA Considerations
IANA has allocated TCP port 7471 for STT. This document makes no
further request of IANA.
6. Security Considerations
In the physical network, STT packets are simply IP datagrams, and do
not introduce new security issues. Most standard IP security
mechanisms (such as IPSEC encryption or authentication) can be
implemented on STT packets if desired. As noted above, however,
tunneling generally interacts poorly with middle boxes, and STT is no
exception. Devices such as firewalls are likely to drop STT traffic
unless the capability to recognize STT packets is implemented, or
unless the STT traffic is itself run over some sort of tunnel that
the firewall is configured to permit. Intrusion detection systems
would similarly need to be enhanced to be able to look inside STT
packets.
It should also be noted that while STT packets resemble TCP segments,
the lack of a TCP state machine means that TCP-related security
issues (e.g. SYN-flooding) do not apply. Similarly, some of the
benefits of the TCP state machine (e.g. the ability to discard
packets with unexpected sequence numbers) are also absent for STT
traffic.
More general issues of security related to network virtualization
overlays are described in [I-D.ietf-nvo3-security-requirements].
7. Contributors
The following individuals contributed to this document:
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Brad McConnell
Rackspace
5000 Walzem Road
San Antonio, TX 78218
Email: bmcconne@rackspace.com
JC Martin
eBay
2145 Hamilton Ave.
San Jose, CA 95125
Email: jcmartin@ebaysf.com
Iben Rodriguez
eBay
2477 Woodland Ave
San Jose, CA 95128
Email: Iben.rodriguez@gmail.com
Ilango Ganga
Intel Corporation
2200 Mission College Blvd.
Santa Clara, CA - 95054
Email: ilango.s.ganga@intel.com
Igor Gashinsky
Yahoo!
111 West 40th Street
New York, NY 10018
Email: igor@yahoo-inc.com
8. Acknowledgements
We thank Martin Casado for inspiring this work and making all the
introductions, and to Ben Pfaff for his explanations of the
implementation. Thanks also to Pierre Ettori, Yukio Ogawa, Koichiro
Seto, Erik Nordmark, Michael Orr and Aibing Zhou for their helpful
comments.
9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[I-D.ietf-nvo3-geneve]
Gross, J. and I. Ganga, "Geneve: Generic Network
Virtualization Encapsulation", draft-ietf-nvo3-geneve-01
(work in progress), January 2016.
[I-D.ietf-nvo3-security-requirements]
Hartman, S., Zhang, D., Wasserman, M., Qiang, Z., and M.
Zhang, "Security Requirements of NVO3", draft-ietf-nvo3-
security-requirements-06 (work in progress), December
2015.
[KM87] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
Proc. ACM SIGCOMM 1987, August 1987.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<http://www.rfc-editor.org/info/rfc2784>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<http://www.rfc-editor.org/info/rfc2983>.
[RFC3931] Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
"Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
RFC 3931, DOI 10.17487/RFC3931, March 2005,
<http://www.rfc-editor.org/info/rfc3931>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <http://www.rfc-editor.org/info/rfc6040>.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<http://www.rfc-editor.org/info/rfc6864>.
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[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger,
L., Sridhar, T., Bursell, M., and C. Wright, "Virtual
eXtensible Local Area Network (VXLAN): A Framework for
Overlaying Virtualized Layer 2 Networks over Layer 3
Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014,
<http://www.rfc-editor.org/info/rfc7348>.
[RFC7364] Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L.,
Kreeger, L., and M. Napierala, "Problem Statement:
Overlays for Network Virtualization", RFC 7364,
DOI 10.17487/RFC7364, October 2014,
<http://www.rfc-editor.org/info/rfc7364>.
[RFC7637] Garg, P., Ed. and Y. Wang, Ed., "NVGRE: Network
Virtualization Using Generic Routing Encapsulation",
RFC 7637, DOI 10.17487/RFC7637, September 2015,
<http://www.rfc-editor.org/info/rfc7637>.
[VL2] Greenberg, A., "VL2: A Scalable and Flexible Data Center
Network", Proc. ACM SIGCOMM 2009, August 2009.
Authors' Addresses
Bruce Davie (editor)
VMware, Inc.
3401 Hillview Ave.
Palo Alto, CA 94304
USA
Email: bdavie@vmware.com
Jesse Gross
VMware, Inc.
3401 Hillview Ave.
Palo Alto, CA 94304
USA
Email: jgross@vmware.com
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