Draft: draft-ietf-quic-load-balancers

Commentary: narrows single - pass encryption to a special - case optimization , and simplifies presentation of the design . 
 There 's a lot of needless bitshifting in the current algorithm . When reading from the right , each byte has to be shifted 4 bits and ORed with the neighboring byte , as currently written . There 's an easier way . When encrypting side X (= { left , right } ) and XORing with side    to zero . Set the index value (= 1 .. 4 ) in the furthest byte that has been zeroed . Encrypt to get ciphertext   with the bits in the same position as    has quietly been some work on tightening up the QUIC - LB specification . At the moment , we are still short on implementations but I am hearing something might happen soon . Anyway ,   has made substantial contributions to the security properties of Stream Cipher CID , which allows smallish CIDs , by making it a three - pass algorithm . We still have the " Block Cipher CID option " which requires CIDs of at least 17 bytes ; AFAICT the only advantage at this point is that it can be decoded with 1 block encryption operation instead of three . In principle , QUIC - LB load balancers can be run with no per - connection state , in which case this would be a per - packet operation . I strongly suspect that real LBs will keep some per-4tuple state , as they do today ; if so , this crypto operation only needs to occur once per packet where the 4 - tuple is new . If so , the CPU impact is vanishingly small except in a storm of garbage packets . So AFAICT , the use case for   is as follows : Willing to run one crypto operation per packet / new 4 - tuple Not OK with doing three crypto operations satisfied with 17B + CIDs I strongly suspect this does not describe a real implementer , and am inclined to simply delete this in my effort to simplify the design . Nevertheless , I 'm taking this to the list in case someone thinks this is an important use case . 
 I am pretty confident about the properties of the stream cipher , but I am not a cryptographer Should we ask a review from the CFRG ? 
 I implemented block cipher cid in my proxy server . It is a bit sad to hear that it is going to be removed from the spec , but I have no strong opinion to against that . If it is removed I 'll just remove the reference to quic lb spec and call it our own . The good thing of block cipher cid is that its implementation is quite easy . 17 bytes cid has good entropy for random bits to manage several cid per connection . 
 It 's not gone yet ! At the very least , we 're going to do the crypto review . 
 FWIW , h2o also uses 17 - byte CID backed by AES - ECB . Our server - id is 64 - bit . While that might sound a bit large , it is good to have some additional space that can be used for detecting broken packets carrying bogus CIDs . Also , we do not retain 4 - tuple state for each connection , because we do not want to retain connection state for all the server cluster . Instead , we decrypt CID of each packet . Our format is different from what is in QUIC - LB ( our design predates the first revision of the draft ) , so we do not mind support for block ciphers getting removed from the document . I 'm just pointing out that there are use cases where block ciphers are sufficient . 
 thanks for the field report . While you 're not conforming to the standard today , I hope you 'll consider h2o servers eventually supporting the standard ( at last optionally ) so they can be used with migration with commodity load balancers . Would the lack of a block cipher make a meaningful difference in your willingness to do so ? 
 if you 've implemented this , please join the # quic - lb channel and we should do interop once the spec settles down 
 Implementing the Stream Cipher CID algorithm is easy for the backend servers . IMO the practical concerns are related to load balancers . Each of the three - pass AES - ECB operation has dependency on earlier passes . That means that it is impossible to run these passes in parallel . My napkin calculation tells me that the CPU cycles required for running Stream Cipher CID algorithm on each full - sized packet would cost like 30 % of decrypting a full - sized packet . There would be at least two ways to mitigate this problem . One is process multiple packets in parallel . The other is cache information , as you 've suggested in this issue . If the load balancer developers are happy to accept this complexity ( for saving few bytes of CID ) , then I do not think I have a reason to argue against . 
 Please do n't make decisions now about the cost of three passes . The CFRG review might very well ask us to increase the number of passes , especially if the nonce is something like a counter instead of a random number . Consider for example the usage pattern in which after a key rotation all servers in the cluster set their nonce to exactly 1 . In effect , that means we are almost down to two passes , and that may arguably be too few . Or maybe not . Review will tell . If I look at the literature , 3 passes is the absolute minimum for achieving an acceptable level of mixing , but there are papers recommending 5 , 6 or even 12 passes . 
 thank you for the caution . Then I wonder if we need to decide the fate of block cipher CIDs , before the design of stream cipher CIDs become more stable . 
 I am not resolving this issue until the crypto review is complete . 
 It 's unfortunately that routing by will meet some problems , image this scenario : The right way SHOULD be that quic - lb route all packets through CID , then the cpu cost ca n't be avoid , but I still approve the usage of stream cipher , from a safety point of view . 
 Good point , I 'll consider this when I 
 reduced the Block CID ( now " single - pass encryption " ) to the special case where noncelen + sidlen = = 16 . 
 Given the work in , I 'm disinclined to remove this special case . Closing . 
 That works . The macros are clever , and you may want to explain the cleverness somewhere . It is obvious to me that you want to align " right " and " left " to an even number of bytes , adding a null nibble at the left or right or at the right of left . That will indeed save a lot of bit twiddling . Found an unrelated typo .
Each configuration specifies the length of the Server ID and Nonce
fields, with limits defined for each algorithm.  When using a given
configuration, the server MUST generate CIDs of length equal to the
lengths of these three fields.
The Server ID is assigned to each server in accordance with sid-
allocation.  Dynamically allocated SIDs are limited to seven octets
or fewer.  Statically allocated ones have different limits for each
algorithm.
The configuration agent assigns a server ID to every server in its
pool, and determines a server ID length (in octets) sufficiently
large to encode all server IDs, including potential future servers.
The Nonce is selected by the server when it generates a CID.  As the
name implies, a server MUST use a nonce no more than once when
generating a CID for a given server ID and unique set of
configuration parameters.
The nonce length MUST be at least 4 octets.  Additional limits on its
length are different for each algorithm.  See cid-entropy for limits
on nonce generation.
Encryption in the algorithms below uses the AES-128-ECB cipher.
Future standards could add new algorithms that use other ciphers to
provide cryptographic agility in accordance with RFC7696.  QUIC-LB
implementations SHOULD be extensible to support new algorithms.
The Plaintext CID Algorithm makes no attempt to obscure the mapping
of connections to servers, significantly increasing linkability.
See cid-format.
On each incoming packet, the load balancer extracts consecutive
octets, beginning with the second octet.  These bytes represent the
server ID.  It ignores the nonce.
When a server needs a new connection ID, it encodes one of its
assigned server IDs in consecutive octets beginning with the second
and chooses a nonce.  This nonce MUST appear to be random (see cid-
entropy).
The Encrypted Short CID algorithm provides cryptographic protection
at the cost of additional per-packet processing at the load balancer
to decrypt every incoming connection ID, unless the load balancer
maintains state for the routing information of any given 4-tuple.
The nonce length MUST be no fewer than 4 octets.  The nonce SHOULD be
at least as long as the server ID in order to save the load balancer
an encryption pass; see below.
When generating a routable connection ID, the server writes arbitrary
bits into its nonce octets, and its provided server ID into the
server ID octets.  See cid-entropy for nonce generation
considerations.
The server encrypts the server ID using the following four pass
algorithm, which leverages 128-bit AES Electronic Codebook (ECB)
mode, much like QUIC header protection.
In the text below, ^ is the XOR function and || is concatenation.
The truncate() function takes the most significant octets of its
argument, so that the XOR function operates on fields of the same
length.  The expand() function outputs 16 octets, with its first
argument in the most significant bits, its second argument in the
least significant bits, and zeros in all other positions.  Thus,
The configuration agent assigns a server ID to every server in its
pool in accordance with sid-allocation, and determines a server ID
length (in octets) sufficiently large to encode all server IDs,
including potential future servers.
Each configuration specifies the length of the Server ID and Nonce
fields, with limits defined for each algorithm.
Optionally, it also defines a 16-octet key.  Note that failure to
define a key means that observers can determine the assigned server
of any connection, significantly increasing the linkability of QUIC
address migration.
The nonce length MUST be at least 4 octets.  The server ID length
MUST be at least 1 octet.
The server writes the first octet and its server ID into their
respective fields.
If there is no key in the configuration, the server MUST fill the
Nonce field with bytes that appear to be random.  If there is a key,
the server fills the nonce field with a nonce of its choosing.  See
cid-entropy for details.
The server MAY append additional bytes to the connection ID, up to
the limit specified in that version of QUIC, for its own use.  These
bytes MUST NOT provide observers with any information that could link
two connection IDs to the same connection, client, or server.  In
particular, all servers using a configuration MUST consistently add
the same length to each connection ID, to preserve the linkability
objectives of QUIC-LB.  Any additional bytes SHOULD appear random
unless individual servers are not distinguishable (e.g. any server
using that configuration appends identical bytes to every connection
ID).
If there is no key in the configuration, the Connection ID is
complete.  Otherwise, there are further steps, as described in the
two following subsections.
Encryption below uses the AES-128-ECB cipher.  Future standards could
add new algorithms that use other ciphers to provide cryptographic
agility in accordance with RFC7696.  QUIC-LB implementations SHOULD
be extensible to support new algorithms.
When the nonce length and server ID length sum to exactly 16 octets,
the server MUST use a single-pass encryption algorithm.  All
connection ID octets except the first form an AES-ECB block.  This
block is encrypted once, and the result forms the second through
seventeenth most significant bytes of the connection ID.
Any other field length requires four passes for encryption and at
least three for decryption.  To understand this algorithm, it is
useful to define four functions that minimize the amount of bit-
shifting necessary in the event that there are an odd number of
octets.
The expand_left() function outputs 16 octets, with its first argument
in the most significant bits, its second argument in the least
significant byte, and zeros in all other positions.  Thus,
expand_right() is similar, except that the second argument is in the
most significant byte, and the first argument is in the least
significant bits.  Therefore,
Encrypt left_0.  The encryption is 128-bit AES-ECB with the key
provided by the configuration agent, and the plaintext argument is
an expanded version of left_0 where left_0 constitutes the most
significant bits, 0x01 is the least significant octet, and all
other bits are zero.
XOR the most significant bits of the ciphertext with right_0 to
form right_1.
Repeat steps 3 and 4, but use them to compute left_1 by expanding
and encrypting right_1 with the least significant octet as 0x02
and XOR the results with left_0.
Repeat steps 3 and 4, but use them to compute left_2 by expanding
and encrypting right_2 with the least significant octet as 0x04
and XOR the results with left_1.
Encrypt the result of expand_left(left_0) to obtain a ciphertext.
XOR the least significant bits of the ciphertext with right_0 to
form right_1.
Repeat steps 3 and 4, but use them to compute left_1 by expanding
and encrypting right_1 with the most significant octet as 0x02 and
XOR the results with left_0.
Repeat steps 3 and 4, but use them to compute left_2 by expanding
and encrypting right_2 with the most significant octet as 0x04 and
XOR the results with left_1.
Upon receipt of a QUIC packet, the load balancer extracts as many of
the earliest octets from the destination connection ID as necessary
to match the server ID.  The nonce immediately follows.
The load balancer decrypts the nonce and the server ID using the
reverse of the algorithm above.
On each incoming packet, the load balancer extracts consecutive
octets, beginning with the second octet.  If there is no key, the
first octets correspond to the server ID.
If server ID length and nonce length sum to exactly 16 octets, they
form a ciphertext block.  The load balancer decrypts the block using
the AES-ECB key and extracts the server ID from the most significant
bytes of the resulting plaintext.
The Encrypted Long CID Algorithm, by using a full 16 octets of
plaintext and a 128-bit cipher, protects the server ID with a single
encryption pass.  However, it also requires connection IDs of at
least 17 octets, increasing overhead of client-to-server packets.
The server ID length MUST be no more than 12 octets.  The nonce and
server ID MUST sum to at least 16 octets.
Upon receipt of a QUIC packet, the load balancer reads the first
octet to obtain the config rotation bits.  It then decrypts the
subsequent 16 octets using AES-ECB decryption and the chosen key.
The first octets of the plaintext contains the server id.
The server encrypts both its server ID and enough octets in a nonce
to form a 16-octet block using the configured AES-ECB key.  Note that
any remaining octets in the nonce are transmitted as plaintext, and
should consider the constraints in cid-entropy.
The load balancer and server MUST agree on a routing algorithm and
the relevant parameters for that algorithm.
All algorithm configurations can have a server ID length, nonce
length, and key.  However, for Plaintext CID, there is no key.
The load balancer and server MUST agree on the connection ID encoding
parameters, which include the server ID length and nonce length.  A
key is optional.
Dynamic server ID allocation is dependent on client-generated
Destination CIDs in Initial Packets being at least 8 octets in
length.  If they are not, the load balancer may not be able to
extract a valid server ID to add to its table.  Configuring a
shorter server ID length can increase robustness to a change.
Note that the Plaintext CID algorithm makes no attempt to obscure the
server mapping, and therefore does not address these concerns.  It
exists to allow consistent CID encoding for compatibility across a
network infrastructure, which makes QUIC robust to NAT rebinding.
Servers that are running the Plaintext CID algorithm SHOULD only use
it to generate new CIDs for the Server Initial Packet and SHOULD NOT
send CIDs in QUIC NEW_CONNECTION_ID frames, except that it sends one
new Connection ID in the event of config rotation config-rotation.
Doing so might falsely suggest to the client that said CIDs were
generated in a secure fashion.
Note that without a key for the encoding, QUIC-LB makes no attempt to
obscure the server mapping, and therefore does not address these
concerns.  Without a key, QUIC-LB merely allows consistent CID
encoding for compatibility across a' network infrastructure, which
makes QUIC robust to NAT rebinding.  Servers that are encoding their
server ID without a key algorithm SHOULD only use it to generate new
CIDs for the Server Initial Packet and SHOULD NOT send CIDs in QUIC
NEW_CONNECTION_ID frames, except that it sends one new Connection ID
in the event of config rotation config-rotation.  Doing so might
falsely suggest to the client that said CIDs were generated in a
secure fashion.
The Encrypted CID algorithms provide robust protection against any
sort of linkage.  The Plaintext CID algorithm makes no attempt to
protect this encoding.
An encrypted encoding provides robust protection against this.  An
unencrypted one provides none.
The Encrypted Short and Encrypted Long algorithms need to generate
different cipher text for each generated Connection ID instance to
protect the Server ID.  To do so, at least four octets of the CID are
reserved for a nonce that, if used only once, will result in unique
cipher text for each Connection ID.
The encrypte mode needs to generate different cipher text for each
generated Connection ID instance to protect the Server ID.  To do so,
at least four octets of the CID are reserved for a nonce that, if
used only once, will result in unique cipher text for each Connection
ID.
Both the Plaintext CID and Encrypted Long CID algorithms send parts
of their nonce in plaintext.  Servers MUST generate nonces so that
the plaintext portion appears to be random.  Observable correlations
between plaintext nonces would provide trivial linkability between
individual connections, rather than just to a common server.
If the nonce is sent in plaintext, servers MUST generate nonces so
that they appear to be random.  Observable correlations between
plaintext nonces would provide trivial linkability between individual
connections, rather than just to a common server.